To me, Mike Glennon is extremely overrated.
I get it. He's got a big arm, he played in an offense that actually resembles a downfield-based NFL offense, and he made some big time plays this year. For stretches of games he looks like Hercules at quarterback. I get it.
But he clearly holds onto the ball too long and has trouble when put into situations where you need to speed up his decision making. It's no coincidence he was right up there for one of the most sacked QBs in college football this year, nor that he had the 2nd most fumbles in all college football this year behind only Taylor Martinez (a running quarterback).
He goes through stretches of games with just awful decision making and execution all the way around. He'll not see a blitz pre-snap, hold onto the ball way too long, make a decision that is really iffy based on the coverage, or just throw a football that (as we saw in the All Star Skills Challenge tonight which he flunked mightily) couldn't hit the broad side of a barn.
When you speed up the game on him in the pros, he's going to fall apart. He is no better a quarterback prospect than Nick Foles was a year ago, and might be considerably worse.
And you're right I don't know why Matt Barkley gets overlooked. Two of the previous (valid) criticisms of the guy were that he's not completing his downfield throws and he's performing poorly in the 4th quarter. So this year he has the best accuracy (by the numbers) of any of the top four quarterbacks at 20+ yards, and he has a 109.0 passer rating in the 4th quarter.
He took two of his weaknesses and completely flipped them around into strengths. And instead of being met with appreciation, people (absurdly and inaccurately) have him slipping out of the 1st round altogether.