Not all the companions are willing to accept a poly relationship, and if you lock in on one who isn’t (like Wyll) everyone else backs off.

(Imagine my surprise when things got serious with Astarion in act 2 and Gale, who I’d never flirted with - I never even got that one scene that starts with the Mystra image - suddenly demanded I choose between them. Awkward.)

And Astarion is just hard to please in general if your character doesn’t find chaos and a little death on the side fun.