Fantastic question. You are right of course. Particularly after the famous manipulation scene of Act I scene 2 in this play, Cassius seems to simply give in to Brutus on a number of issues where he has very strong views. The first of these is of course in Act II scene 1, where Brutus argues against the killing of Marc Antony along with Caesar and insists that Marc Antony is actually a harmless figure who is given to drink, dissipation and other frivolous activities:
Alas, good Cassius, do not think of him:
If he love Caesar, all that he can do
Is to himself, take thought and die for Caesar:
And that were much he should; for he is given
To sports, to wildness and much company.
Even though Cassius does make one other effort to try and persuade Brutus that Marc Antony is dangerous and needs to be dealt with, at the same time, after this he stops and raises this issue no more. It is interesting that, although Cassius is such a canny character who shows his ability to manipulate so easily in the seduction scene, he gives in so easily at this stage in the play. Perhaps it is that he is able to exert his power best behind the scenes. Now that he has recruited Brutus to the cause, Brutus seems to naturally take a leading role and Cassius, as a behind-the-scenes kind of character, is left to play second fiddle to him.