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.