It comes down to a variety of reasons...
During the 70's and early 80's there was a lot of runners coming together and training together a lot. There still was not a whole lot of money in the sport and guys wanted to continue training at a high level after college to really explore their potential. Plus, they loved to run and compete.
During the late-80's corporations, specifically the shoe companies, started to sponsor individual runners and the top runners would either be isolated or isolate themselves and these training groups that were all over the place started to split apart. Then with the sport becoming more money driven, guys coming out of school thought that they needed money to continue to compete.
I think a lot of it is that in today's world guys coming out of college feel the need that they need a group to run with or a sponsor. There needs to be an incentive out there to continue running at a competitive level. I think with the various groups popping up all over the U.S. the depth this country has had in the past will be surpassed greatly.
There are some great groups out there right now that get little recognition and little financial support, but are doing a great job. One comes to mind in Team Good River. The Boulder Distance Project is also a good one. I think more and more of these types of clubs will come about over the next three to five years and that will create some very impressive depth.