Because his arm looked like it could have been stuck in a position where he couldn't actually tap, and his other arm was wrapped up. Sometimes if you're out your arms do strange things, for example Miesha Tate finishing Holly Holm.

100%. I have more chance of fighting either of these two than them fighting each other.

On the replay it kinda does look like he tries to tap but his arm is stuck so he can't, but then when Keith Peterson moves in and touches his hand it's clear that he could've tapped if that was his intention. Poor split-second decision by the ref there.