I believe keirin frames "exploded" for a few reasons:

1) Tough, strong, simple frames. They hold up to abuse well, and can do double duty on track and street. The NJS specs also lead one to believe that they can withstand and hold up to crashes, abuse, etc, since they are "made" for that.

2) Aside from getting a custom frame, you can't quite find this aesthetic (particularly lugged) in the US. And with the craftsmanship that goes into making a keirin frame? Not to be found off the shelf.

3) E-FRAMEBANK. I think, at least for people on this forum, that helped a lot of people get a lot of sexy frames from classic Japanese builders into their hands for the price of a Soma or Surly or other easily gotten frame. From there, it was just a matter of "Hey, why not build this up all Japanese?" And thus started the NJS/Japanese fetishism for parts.

4) The "coolness" or "hotttness" factor. Yep, come on - it's a sexy frame/bike. It has appeal and people dig 'em. They have cachet.

5) Back to aesthetics - they're minimal. They appeal to a certain aesthetic. Less is more. Less can hold up better. Less can be beat up. You don't worry about it. Sorta like the whole fixie thing.