Stephen White of SB Nation does phenomenal draft breakdowns every year and he NAILED why Andrew Billings wouldn't be a top selection in this piece.
In short, Billings' dominance against the run isn't valued as highly in the NFL particularly when his shorter reach doesn't translate to him being able to 2-gap and control the middle. If he had more on tape to indicate he could be a good pass-rusher he might have gone higher but alas, it's not so.
Rough deal for Baylor, who now not only lose arguably their best player from 2015 before his eligibility was up and not only don't have good replacements lined up, but now they don't have a high draft pick result to sell to future recruits.
Urban Meyer isn't terribly worried about losing half his depth chart to the NFL because he knows it makes it that much more likely that he can reload. For Baylor that's a slightly tougher sell with every spot Billings drops. Their switch to the 3-4 combined with their ability to sell the top offensive players in the state means they have nothing to worry about here long-term, though.
The question I'd love to tackle here though is this: How could a player that was able to dominate games in the spread passing-heavy Big 12 due to his run defense could be so overlooked by the NFL?
The answer is the prevalence of run/pass options in college football (RPOs). They make all the difference.
In the NFL, RPOs are increasingly common but they are limited by the fact that OL can only get one yard downfield before a pass can be thrown beyond the line of scrimmage. In the NFL RPOs are usually quick hitters and aren't a major part of the offense. What is a major part of NFL offenses is the drop back passing game, and nose tackles aren't that helpful in stopping those.
In college, OL are allowed to get three yards beyond the line of scrimmage on RPOs and thus these schemes are dominant and major features in Big 12 offenses.
Big 12 teams hardly ever run the ball without attaching quick pass options on the outside for their QB, which often has the effect of forcing defenses to vacate the box and stop the run with only five or six defenders.
In that world, having a nose-tackle who can beat double teams and negate the disadvantage of moving linebackers out of the box to stop quick passes is INVALUABLE. If opposing teams can't run on five in the box and have to try and beat you throwing quick routes on a spread out, cover 2 defense they aren't going to get very far. That's tough sledding.
The result is that a guy like Billings, who often dominated interior OL in the Big 12 and allowed Baylor to flood the field with quick, spread out linebackers, may not have as much value to NFL teams that are trying to figure out how to disrupt the timing and throwing windows of a spread, West Coast passing attack.
Hopefully some team still drafts him for his awesome potential as a run-stuffer on standard downs, he'll probably still be a good player but this is why his value doesn't translate cleanly to the pro game.