It’s two things, I think.
One is that over here, people are usually anonymous, so can speak with a certain candor you won’t find in the real world. Myself, for instance, the times I’ve brought up the Moshiach question with Lubavitchers was always uncomfortable for both of us and the discussion was quickly dropped. And I’m afraid of losing friendships and alienating people when I find out that they worship a dead man, or they find out that I hold of them as an apikores.
A second reason could be that the Messianism and Rebbe worship (yes it happens, yes it’s common, no not in the way other Chassidim respect their Rebbe) is what really pushed Lubavitch out of the rest of the frum world. So when a frum person sees a Lubavitcher, one thought is “Huh, this person could really be part of our community but there’s that huge stumbling block in the way” and it really drives the whole relationship. It’s the reason why you won’t find too many mentions of Chabad events in Yated or Hamodia, or why Chabad Rabbonim aren’t speaking at the Siyum HaShas. It’s the 800 pound gorilla in the room and we can’t just ignore it.