Because the "scan" doesn't work unless they are holding perfectly still. If they are moving on the other side of a wall that it scans then they won't show.
I may use the MMS sight once in awhile but that is because I it's perfectly clear sight picture & nothing do with how it may outline someone once out of every players. --- When I read about the evils of drinking, I gave up reading.
I believe the scan doesn't see them through walls unless they're stationary. It seems to pick them up through smoke even when they're moving. I'd say it's also situational, but lots of things are. It's really funny to silenced-KSG someone through a cloud when their outline is staring right at you, completely oblivious.
I really don't like it because of the constant wave that the sight does. It makes me lose my enemies in the sight a lot of the times. I might try running PDW MMS Stock and Silencer with Scavenger and Smoke Grenades. --- There is no pleasure in life. There is only a relief of pain. We have to suffer to be on top.