Because they want the property (think of all the old churches in ECUSA that have valuable real estate, huge endowments, etc.), the prestige, and the power.
There are already churches in existence that have no problem with homosexuals -- the Metropolitan Community Church and the Unitarians, just to name two -- but that's not good enough.
I also think that the ECUSA activists want to take over the church of their childhood. They think if they can just do that, it will silence that still small voice that is whispering to them that what they are doing is wrong. It won't of course, but I think it's that just as much as the money.
I think you hit the nail on the head here.