Because it's NOT the real issue. The real issue is public recognition, then tolerance, then finally acceptance of homosexuality as an equally alternative lifestyle.
Same-sex marriage is one part of that process. Non-discrimination against homosexuals is another. Hate speech protection is another. Educating our children on homosexuality is another. Homosexual adoptions are another. Research into the genetic background of homosexuality is another.
Each are pieces of the whole.
Money is the real issue. Look around, homosexuality is currently treated as an "equally alternative lifestyle." Just try denouncing it and see what happens to you.
What they want is the right to tax breaks and survivor benefits.
This is a states issue and needs to remain a states issue.