When I first started torrenting, it was because of the hype of course. In a nutshell, it was essentially because of WTAW. When you're a newbie, and you end up in a place like this, you really don't know any better. You see a review for some 1337 tracker, let's say FSC, that's about 3 years old, doesn't have many pictures, and goes on and on about how the community is the best on the net, the forums are an utopia for discussion, etc. The reality upon joining is quite different, but by then it's too late.
As time went by (as has already been said), these invites are eventually just offered to you. When someone comes along and says "hey, want to join uk-t?" out of the blue, it's hard not to be curious about the site.
I could count on one hand the number of smaller trackers I've joined that have lived up to expectations in any meaningful way, and usually they're niche trackers, not these small 0-days with "legendary" communities. Smaller and more secretive just means fewer seeds, worse torrent retention, less content, dead forums populated only by spamthreads, IRC channels with about five people who ever talk, etc. Over time I've realized that it's all pretty useless, and all of these sites have the exact same core of members who you can interact with at any one of them.