If it happens - at that is a HUGE if - it will be because manufacturers figure out how players can wear pads without them being constrictive or restrictive. And that is something that has been needed since the inception of padding.
If it happens, it will probably trickle down to the NCAA. The NFL is successful in enforcing their uniform and equipment rules, because they hit the players in their pocketbooks for violations. The NCAA can't really fine players, and they haven't shown any backbone in moving to fine the institutions. Until they do, knees will remain uncovered.
Of course, the NCAA may not exist as we know it in a few years, anyway. If a 'big' conference commissioner has his way, a 'select' number of teams (64-96) will separate from the NCAA, and form regional super-conferences, with a playoff system. They'll write their own rules, and my guess is they'll play a game that more closely resembles the NFL than traditional college football, because it is more conducive to revenue generation.