It had better not be unavailable prerequisite courses, especially since there's no excuse during that period for insufficient lab hardware resources (not true in the bad old days before most everyone can afford a laptop more powerful than a Cray 1). That's one of the reasons for paying the big bucks to go to a private school, and why many state schools are not so much the bargains that appear to be if you
can't graduate within 4 or even 5 years.
I can't speak about CMU, but I know there are basically no limits to what MIT will do to make sure every major can graduate in 4 years, and almost always take the courses they need when they want to take them. That has sometimes meant non-majors couldn't take courses intended for majors, but otherwise....