For the same reason that a movie like Napoleon Dynamite becomes iconic for a sliver of a generation. It's just a perfect moment in time for someone to capitalize on something. Tom Green was the right balance of surreal stupid comedy when America and Canada desperately wanted surreal stupid comedy; People wanted reality TV, and it had a mix of reality TV in it. Just after it shifted from him to Jackass and CKY, which had elements of TOm Green's physical stupid comedy and wratched it up to 100, and then (thankfully) killed it dead.
In 3, 5, 10, and 20 years we might look back at the most famous YOuTube stars and think, like "How the fuck did these people possibly get famous?" We can already do it with "Prank Videos." The meteoric rise and swift fall of prank videos were like this perfect moment in time for whatever YouTuber jumped on it earliest and best, milked the fuck out of it, and now they're making videos about how prank videos are over.
I actually think Tom Green has real comedic talent, he's always been a really good interview even decades after his show, which shows that he's not just a one-hit-wonder.
My favorite bit of weird 1990s pop trivia is that WWF toyed with this segment called "GDTV" in the late 1990s, where these short vignettes of "behind the scenes" stuff in WWF would be shown on a closed-circuit like monitor. Originally it was going to be Goldust's re-introduction to WWF after he had fallen off the wagon, left, and was coming back. But then he fell off the wagon again and they changed the segment to GTV, with the producers (probably Russo) intending to introduce ... Tom Green ... as the man behind GTV. Thankfully, Vince watched some segments of the Tom Green show, said (rightly), ... this isn't funny .. and completely axed the angle. We were ultimately moving to a disastrous synergy of Attitude Era and Tom Green, and it was narrowly avoided by Vince McMahon.