Moments in time. There’s a great article here at Louder Sound dot com that really sets up the backstory of the performance. There’s a YouTube channel (Wings of Pegasus) that breaks it down musically. Why all the attention to one song? It changed their life and was the forerunner for a whole new style of music that would be coming in 10 years, punk rock / new wave / alternative rock that was coming. The times they were a changing. How momentous the whole thing was is evidenced by the title quote of this post. A rain delay had put their performance on in the middle of the night. Relatively no one saw it. It was only a year later when the Woodstock movie came out that it put the group and the performance on the map. This sound wasn’t heard in 1969. It was new. Plus you got a British white guy singing the blues as well as anyone. What I like about the article was how it tells about bands after that who didn’t want to come on after these guys. They were so high energy and advanced musically they made the next group on look bad.
““You had police with guns, and cotton wool in their ears, sneering at the band and looking for half a chance to beat up the audience. It was awful. It had all gone wrong and I was thinking, what the fuck am I doing here?”
And when that Woodstock movie put them on the map, and their careers should have been put on cruise control, and the previous 10 years of apprenticeship pain should have been forgotten, what destroyed it all? What always does with musical groups: Ego. Alvin was the singer, songwriter, lead guitarist, he got all the attention. And drugs and alcohol. Same story, different people. All the things that could have been. “Initiate self-destruct sequence.”
Before the collapse Alvin had become the typical prima donna and vetoed the follow up to their mega hit I’d Love to Change the World, with Tomorrow I’ll Be Out of Town. Record exec Clive Davis (who later guided Peter Frampton’s stellar career) said to hell with you then, and they were soon “Johnny Bravo’d” (they found somebody else to fit the suit). 2016 interview with bassist Leo Lyons.
[‘I’d Love to Change the World‘]
Iowa life 
I used to love Ten Years After, and I always wondered why they didn’t bring out more albums. They just kind of dropped off the map.
Yeah I didn’t see their Woodstock performance until a couple of years ago on a TCM replay. Woodstock had a lot of pretentious acts of questionable talent, then these guys get nothing! The ego it takes to be great, took so many of them out.