Because Terra wasn't the sort of person who would ever even entertain the idea of doing some of the things Xemnas did.
I'd be perfectly happy with him influencing Xemnas -- being the reason Xemnas never used a Keyblade, being the reason Xemnas sat around in the Chamber of Repose most of the time, being the reason Xemnas was looking for the Chamber of Waking etc. But the point of Terra's character in Birth by Sleep is that he's this sweet and naive guy who's broken and used because of his inability to live up to his Master's unfair black-and-white expectations. Terra's decidedly not Riku, who grudgingly does questionable things for the sake of his friends; he beats himself up over every perceived failure on his part to reach the highest moral standards, even when he didn't really have much of a choice in the matter. Terra, by the end of the game, wanted to die so Xehanort couldn't use him anymore.
Xemnas, on the other hand, has none of these traits. He's cold, ruthless, and only interested in gaining Kingdom Hearts, just like Xehanort. He shows no affection for anyone, including Roxas. He orders a fight to the death between Roxas and Xion (who may or may not have appeared like Ven to him at some points) just because. He wants to use Kingdom Hearts to create a new world, which was Xehanort's real goal to begin with.
If Xemnas isn't controlled by Xehanort, he did exactly what Xehanort would have done. And, even if we're assuming a vengeful, morally-broken Terra, that would be the last thing he would ever want.