A few reasons I can think of:
1) Better education and higher income levels mean that homosexuals are good at organizing politically and have the financial means to do so.
2) Anti-gay discrimination has lead to the creation of various activist groups that work to protect the interests of homosexuals.
3) Geographical clustering- large numbers of homosexuals live in places that are proximate to power: New York, Washington, Los Angeles. Living there gives homosexuals a greater ability to influence policy than if they lived, for example, in Idaho.
In addition to that, someone somewhere posited that the high percentage of gays in DC has put them in contact with many policymakers. Putting their best foot forward (ever hear of a "pride parade" in DC?), they have been schmoozing decisionmakers assiduously for 30 years. As a result, many policymakers -- most spectacularly including the current "conservative" President -- have been influenced to adopt "gay-friendly" attitudes and policies w/o regard to further consequences of their doing so.
We can expect more of the same. Schmoozing and good behavior in DC, chickenhawking, chicken-skinning, and open predation in Burbland.