Because the organizers are afraid that Palestinian gays will speak about the other violence: the violence of the racist Israeli state towards its Arab inhabitants, gay or non-gay, and the violence of the Israeli occupation that victimizes thousands that go unmourned.
Also gay Palestinians may challenge the myth Israelis embrace that Israel is wonderful to Palestinian gays because it's the only democracy in the Middle East. They may tell stories of how Palestinian gays are exploited by the state to construct this myth but how little actual support and help they get.
I also wonder if these kinds of gay Palestinians, the Aswat women kind, are not the gay Palestinians the Israeli establishment likes to parade around. They are gay Palestinians who speak for themselves, they are activists in their own communities, and they have courage and spunk. They don't fit the role assigned to them by the Israeli state and the gay community of either being victims of their society or native informants.
But ultimately gay Palestinians would ruin the anti-homophobic demonstration, which was apparently full of homophobic politicians, by injecting politics into the proceedings. Yes, this was meant to be a feel-good event which had no place for "politics."
As if gay rights and anti-homophobia struggle can be apolitical! Only in Israel.
(ps. my only objection to the article I link to in this post is the hijab metaphor. It is neither necessary nor appropriate).