I as a player, don't want to go pick a fight with the Head when we haven't even had a proper dust up with the Incubators. It's annoying. I'd much rather have a solid lead on how to deal with them first, and then promise to take the Head's head.
I… the incubators are a probably universal scale alien race with all the power and level of technology that implies, the Head is far, far easier to deal with.
We're not fighting Kyubey, we are lucky it doesn't want to fight, it is easier to convince it or con it using it's own system than to remove it's influence from the entire planet, and we already have a thing that points in that direction: the increase of exposure level it got, so we actually do have a way to work on the Kyubey problem.
Besides, you really think that Angela wouldn't help us against the incubators if we asked for help and would require a previous promise to do so?
It's already part of the deal, even when not said.