There is no a priori reason to think that the efficacy of charitable giving should have any relation whatsoever to utilitarianism. Yet it occupies a huge part of the movement.
I think the argument is that, a priori, utilitarians think we should give effectively. Further, given the facts as they far (namely that effective donations can do an astronomical amount of good), there are incredibly strong moral reasons for utilitarians to promote effective giving and thus to participate in the EA movement.
I think that [the obsession with utilitarianism] is regretful… because it stifles the kind of diversity which is necessary to create a genuinely ecumenical movement.
I do find discussions like this a little embarrassing but then again they are interesting to the members of the EA community and this is an inward-facing page. Nonetheless I do share your fears about it putting outsiders off.
I agree completely that talking with people about values is the right way to go. Also, I don’t think we need to try and convince them to be utilitarians or nearly-utilitarian. Stressing that all people are equal and pointing to the terrible injustice of the current situation is already powerful, and those ideas aren’t distinctively utilitarian.