The consensus is that there isn't enough room, at least on the part of the mountain where the other portraits are. Usually this ends the discussion. Usually. Putting Reagan on the mountain was proposed during his presidency, and the idea was shot down by a terse prnouncement that there wasn't room up there for anyone else, no matter who they were.
Gutzon Borglum, the artist who created and supervised the work, had to make quite a few changes just to get the four men up there. If I remember right, Jefferson got the worst of it. His position and the direction of his gaze had to be adjusted to work with the stone that was there. And even then, his face has a serious crack in it.