It's a good question, because there were quite a lot of festivals in the late 60s, and many in just 1969 alone. The Denver Pop Festival, the Atlanta Pop Festival, the Seattle Pop Festival, the Atlantic City Pop Festival, the Texas International Pop Festival, and many others all took place between June and August 1969.

There are a few others that do get some attention from that same year. The Isle of Wight Festival marked Bob Dylan's first live performance since 1966, so it was highly anticipated, though its importance might be a bit deflated since Dylan has been performing live regularly ever since, including his decades-long endless tour that's still ongoing. There was also Altamont, which the Rolling Stones played at, notorious because a fan was killed. And there is also the Toronto Rock & Roll Revival, most famous because it was the first truly live performance by a Beatles in front of a paying audience since 1966, and John Lennon & The Plastic Ono Band released their set as a live album (Live Peace In Toronto).

Still, they all pale in comparison to the fame of Woodstock. There are arguably several reasons for this:

1. The Woodstock concert was released as a film. The film was released under the title Woodstock and it won the Academy Award for Best Documentary Feature in 1970. So, unlike many other concert festivals of the 1960s, many people who did not attend actually got to see it. Well, parts of it anyway.

2. The concert was also released as an album. It was a triple-album, released in 1970, and reached #1 in the Billboard album charts. According to the RIAA's website, the album was certified Gold on May 22, 1970, less than two weeks after being released. It was certified Platinum in 1993, and has sold 2 million copies. There was a follow-up album, Woodstock Two, released a year later in 1971, and it peaked at #7 and also went Gold. Some of the various individual sets by the performers have also been released in full, notably Jimi Hendrix's, though not until the 1990s.

3. A ton of people attended. An estimated 400,000-500,000 people ended up showing up. In comparison, Monterey in 1967 is estimated to have been attended by 25,000-90,000 people, but the higher figure is only taking into account the number of unique attendees over the course of its three days. Many people only attended part of the festival. Atlanta, Texas, and the Isle of Wight festivals were among the other biggest festivals of 1969, but the high estimates are that they only reached 150,000 attendees each. Altamont is the only one that came close to Woodstock, but still only topped out at 300,000. And all those had enough contingencies in place that they could handle the crowds, more or less (well, Altamont has its own story). Woodstock was a complete disaster. People were stuck there once they got there, and it took days for everybody to leave. Which leads to another reason it's remembered.

4. It was an organizational disaster. The organizers were unprepared for the crowd that showed up. They were unprepared, period. Other festivals that had happened that summer, and in the years prior, had been very nice and orderly. The only other festivals before it that were remotely as notorious were the Trips Festival and Acid Tests in California in 1965-66, which several San Francisco bands had played at (most notably the Grateful Dead), but they were much, much smaller, and only notorious for the (then-legal) use of LSD. LSD remained pervasive at many other festivals after it became illegal, but they still went off without a hitch. The Monterey International Pop Festival was particularly influential. The drug use was rampant at it, and there was an expectation that the festival might turn disastrous since it was the first major festival in California the summer following LSD being made illegal, but it ended up being the most well-received rock festival ever, up to that point in history. That's why all those 1968-69 festivals were called "[City] International Pop Festival" because they were trying to be "the next Monterey". Like Woodstock after it, Monterey was filmed, so a lot of people were able to see Monterey, too, even if they didn't attend.

Woodstock was supposed to be yet another Monterey, nice and orderly, with lots of good music and drugs, but it ended up being completely different. Almost half a million people were stuck in the mud and rain for almost a week. There was trouble getting the performers to even get on the stage, and not just because of the crowds, but because of the money. The organizers had promised many of the performers money that they couldn't deliver on. There was a bunch of downtime throughout the festival, especially early on. It almost didn't happen in the first place, because the original site couldn't be used due to a local ordinance passed, specifically to prevent it. After a couple other sites fell through, it ended up being moved to a local farmer's farm, who was feeling generous. The festival went on. It lent itself to a mythology, since there was so much backstory to tell.

5. Because of the chaos, it was news as it was happening. For instance, here is the CBS Evening News report that aired on the last day of the festival. There were newspaper articles about it as it was going on. But perhaps most significantly, the day after the festival—or, later in the day that Hendrix performed "The Star-Spangled Banner"—several of the performers at Woodstock performed on The Dick Cavett Show, and gave interviews. You can see in that clip Stephen Stills, David Crosby, and the Jefferson Airplane. Stills is even still wearing his same muddy jeans. They give their take on what had just happened, and the whole discussion reveals that it was already being talked about as a significant pop culture moment.

It was also aided by the fact that it took place in the backyard of New York City, the center of American news media. Even Altamont and Monterey weren't exactly a leisurely drive away from Los Angeles, and despite San Francisco being a major city, it's not like New York City or Washington, D.C., when it comes to media coverage. As a result, the New York press wrote about the fallout from the event at length. There had been some coverage before, too, but it was more significant during and after. In the essay "Reporting Woodstock: Some Contemporary Press Reflections on the Festival" published in Remembering Woodstock, author Simon Warner explains:

"The New York Times, the daily newspaper of the largest city located in the state where the Woodstock festival took place, provides the most substantial reflection on this period. Between 15 August and 24 August 1969, it covered the festival in a range of stories - news articles, features, editorials and readers' letters. Reports ranged from pre-festival anxieties over a decision to prevent New York City police officers performing security roles in an off-duty capacity, to a post-mortem in the Sunday edition the weekend after the festival, which gave an almost unanimous thumbs-up to the gathering.

"In between, the tone of the reporting and comment fluctuated in style and message. While the lengthy news accounts which filled column after column from Saturday 16 August through to Tuesday 19 August presented a measured description of the chaotic scene surrounding the rise and fall of the three-day celebration, the two key editorial responses contrasted significantly in their expression. One letter from a reader eulogized the festival in its wake, while a post-festival news item, which reported comments made by locals, communicated a considerably more hostile feeling towards the event."

Warner goes on about the New York Times press coverage, as well as other coverage from other sources, particularly Rolling Stone who was supportive, describing it in terms of the emerging youth culture that they had already been talking about in their magazine. And that leads to a last point.

6. Almost immediately upon reflection, it was discussed as a cultural milestone for the Baby Boomers. The Baby Boomers and the emerging youth culture, characterized as "hippies" (even if that wasn't always true), had already been given much discussion in American culture up to that point. Woodstock was seen as something of a "coming out party", so to speak. They were a force to be reckoned with, and while there was some negative coverage and discussion of Woodstock among older Americans, it marked a point where the Baby Boomers, as a generation, could not be held back. The whole event was sort of an analogy for it. The adults in actual Woodstock, New York, had made political moves to get the event cancelled, or at least moved, and the organizers ended up with a last-ditch third option, far from ideal. And it happened anyway. In the mud and the rain. And it's all that anybody was discussing. Half a million of the Baby Boomer generation had shown up despite the "parents" trying to stop it, but it couldn't be stopped. And this was the generation who would be voting and taking over politics and business in the 1970s and beyond. There was much talk about who the Baby Boomers were as a people already, and what the future held as this massive generation reached adulthood. In retrospect, a lot of the characterization was probably wrong, and the characterization may have even created a backlash among the Baby Boomers themselves (they are a much more conservative generation than the 1960s media coverage assumed they were). But at that time, Baby Boomers and the types of young people who chose to attend Woodstock were synonymous in the public mind, so it was recognized as a cultural milestone for the Boomers almost immediately.

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Yes, thank you