It is possible that there's plenty of reporting of Muslim bad apples, but that it is carefully and properly kept quiet. I hope so, though it suggests better skill at keeping secrets than I've come to expect. Perhaps the lower echelons are better at it--keeping certain top people in dark, maybe? Would you tell Biden anything you didn't want to see in the newspaper?
If you're going to take the risk of reporting someone suspicious, you want some confidence that the report will be taken seriously. Fecklessness at the top (so Iraqis think we're supporting Daesh? I can see why they'd think so), and CYA cultures all over the place--I'd wonder if a tip would go in the file and forget bin. But maybe this agency is different this time.
Someone suggested that a reason more US Muslims don't report suspicious behavior (at least that we hear about) is fear of retaliation--one thing jihadists hate more than non-Muslims is "bad Muslims," and contrary to the party line it smells like quite a few of these lone wolves aren't that alone. (I also see polls claiming that a non-negligible fraction support the jihadis, but I've no idea how accurate they are.) I doubt that most jihadis stand up in the mosque and call for war--unless the whole mosque does; suspicion rises on more subtle criteria: non-actionable criteria.
So we're going to keep getting "yes, they were on a list" after-the-fact. And no doubt CAIR has a press release warning against a backlash after next Wednesday's attack.