Because, on one hand, that ending was *perfect*. Everything was tied up except for the things that shouldn't be. A sequel can't possibly improve upon that ending.
But on the other hand . . . more Psychonauts.
You know what might work? Set the next one ten or twenty years in the future, so it's not happening *right after* this one. That could work. As it is . . . I think everything's neatly enough packaged.
Incidentally, go buy Psychonauts.