I think there are few factors.
1) Since many female runners excel at such a young age (and may not progress that much), 8+ years of training can take a mental and physical toll. It may just not be worth it to continue.
2) Many times the coaching at the collegiate level is done by men, which brings into play the weird male coach/female athlete relationship. Every male coach, to some degree, is at least a little flirtatious with female athletes. Sometimes it gets a little strange . . . for some people, by the end of college enough is enough.
3) Other priorities (the previously mentioned Type A personality)
4) Not enough advertisement of sub-elite post-collegiate training groups. Someone who runs 16:30-17:00 may think they're done unless they move to road racing after college - that's not true ! (this holds for men as well).