Most defenses have no idea how to stop the Spread Attack and teams that run uptempo.
Defenses like Alabama and FSU are able to do it because they dominate the line of scrimmage.
To stop the spread attack you have to be able to handle the running game with a short box. By that I mean 6 defenders in the box. FSU plays a Nickel Defense with 5 defensive backs and 6 defenders in the box to stop the run as its Base Defense. That allows you to commit extra defenders to cover the 4 receivers that are spread out. If you have to commit 7 or 8 defenders to the box to stop the run then you have to lock up man to man and in many cases you have to do it without Safety help. Most college Corners simply can't do it. It's a numbers game. The really good defensive teams have great Defensive Linemen that are hard to block and they have Corners that can play Man to Man coverage. Spread Offenses will kill Zone Defenses.
So giving up 50 points is kind of the norm. There are a handful of defenses that can lock down. Namely FSU, Bama and Florida I think are the best defenses in the country.