What an interesting question! Okay, I'll take a shot at it... :-)

There have been several attempts at "another Woodstock," but can there ever really be another one? Does calling an event "Woodstock," hiring some of the same bands who played the 1969 event, or hosting it on the same property really mean the festival is "another Woodstock?" I don't think so ... I wasn't there, but the original festival has always fascinated me as a great moment in rock history and over the years I've researched quite a bit to discover what it was, and what it meant to the people who were really there.

RE: the question, "what specific events led to the decision of ‘no more?'" To me, the decision of "no more" is not so easy to attribute to any given event or any human being's choice on whether to host a "Woodstock." The top reasons I believe there could never be another Woodstock have everything to do with cultural zeitgeist. I believe Woodstock belongs to those so often referred to as "the Woodstock generation," for whom the event was nothing short of revolutionary; it was so much more than just a chance to camp out and hear some great bands. Though the fest was put on by investors (who hosted it with a profit-making motive), the majority who attended and participated didn't seem to perceive it that way. It was a celebration of youth and an idealistic counterculture that was "of the moment" in 1969.

Okay, top five reasons...

1) Some things can only be authentic once.

When Woodstock happened, it was widely perceived as being a music festival and cultural gathering that would be unlike anything that had ever happened before. That has a lot to do with why it was special. Any event that tries to "recapture" Woodstock is already putting their reference point as a past event, a sense of nostalgia vs. coming up with a new idea and hoping it might be revolutionary on it's own terms.

2) Overt corporate sponsorship and genuine "counterculture" make strange bedfellows.

One of the later "Woodstock" events was sponsored by Pepsi. Today, we are accustomed to the idea of major corporations sponsoring music festivals, and even having a strong presence with their exhibits and promotions DURING the festivals. We accept this, and know most big fests couldn't happen without major corporate funding. This is not at all a "Woodstock" point of view. Though the original event was put on by investors with a profit making motive, nothing about the festival promotion and nothing taking place AT the festival acknowledged this to the masses.

3) Perception of entirely altruistic motive on the part of participating bands vs. profit motive

Sure, the featured bands in 1969 got paid for playing and many might have had financial motives for being there ... but in the moment of actually being at the festival, they weren't perceived that way by their audience. There was a widespread belief that the bands were there for the same reason as they audience, a common belief in music and youth culture as vehicles for social change. Nowadays, we all know that most bands take fest gigs more for promotional reasons vs. altruistic motives. Consider Trent Reznor's comments to Rolling Stone re: his participation in the Woodstock '94 event: ""At the Woodstock I did all you heard about was the Pepsi logo on the f*****g bird thing and how it was all about money. Bands were getting shit-canned for doing it for money..."

4) "Be here now" vs. the era of "virtual" experience

The 1969 event was filmed, and there were some journalists there to report on the event; however, their presence in the eyes of the audience was minimal by today's standards. The event was perceived as being all about a group experience that could only be shared by those who were there ... you were either there, or you weren't. Nowadays it would be next to impossible to have that vibe at an event. People who attend music fests want to tweet about it, youtube about it, and facebook about it at their earliest possible convenience. "We" have become a culture that lives in two worlds simultaneously ... I think the commitment to "being there" now has a very different quality that it had back in the days before the internet. The people who attended Woodstock 1969 were wholeheartedly "there," and had little means to concern themselves about what the world outside saw of their experience until it was over.

5) Major music festivals have become common...they're fun, but we don't expect them to start a revolution ;-)

We now have quite a few awesome major rock and alt rock music festivals in America ... they're fun, but we don't attend them with the thought that they are ushering in a bold new era of peace or otherwise changing the world just by virtue of thousands of music fans getting together. I kinda think of great fests like Bonnaroo and Coachella as the grandchilden of Woodstock ... and it's just as it should be that the grandchildren are part of another generation, and have a message all their own ;-)

1. We would need Neo Hippies
2. We would need some huge names. I don't think that the stars shine quite as brightly as they used to. Anyway, do you think that the top artist of today match what we had in the late 60s?
3. We have had new Woodstock concerts since the first one. Did they have as much impact?
4. You would need to duplicate the same conditions that brought Woodstock about. It was a counter-culture event.
5. Obama is no Richard Nixon.

The second Woodstock was so bad that...well, I was on the radio doing top 40 from 1990 to 1993. These would have been bands of interest to me. I t was in 1994, I wasn't there, but it was so non-memorable. that the big story was the mud. (see top pic)

1. I don't know if New York State can produce that much mud again.
"Unlike the media portrayal, the majority were not among the "mud people". I was a mud person for a couple of hours Saturday afternoon and it was great while I was rolling in it, but the mud feels weird once it dries up. The storm washed me off and I didn't go back. The mud people had a direct line to the front of the stage---people would separate whenever they wanted to go through. There were a lot of boys who were naked and a lot of girls who were top less. I have thus included one obligatory picture. (of a topless girl, not shown here) "
2. Violence:
"Things got really out of control as audience members climbed up on stage and security people tried to stop them. It was a futile effort; there was total and utter chaos. One little kid ran on to the stage, hugged Armstrong, and started running away, but the singer chased after him and brought him back to sing a few lines! I bet that made that kid's day. Running around on stage had its problems however, as (Green Day) bassist Mike Dirnt discovered. As the show ended, the security people mistook Dirnt running around for one of the stage crashers and tried to kick him out, very much against his insistence that he belonged to the band:

The caption to the bottom pic reads:
"Hey man! I'm the bassist!"
"Yeah right, we've heard that one before."

The memoir goes on:
"It was the most out-of-control show I saw, yet nothing violent happened and the security people and the audience made peace afterward (though I hear Dirnt lost 3 of his teeth). "

Losing 3 teeth isn't violent?

3.- The drugs have changed and are more varied and deadly.
4. -No one thought about STDs in the 60s. (or likely in the 90s)
5. Who can take time off or spend the money these days?