It was a fraction of a second and the fight had just started.
JDS is very strong and has good TDD, he is also larger than Cain and was still fresh. If he over committed to a takedown that was not set up well he could have burned himself out, failing a takedown you work hard for is also mentally defeating and gives encouragement to your oppenent.
The fight was like what? 60 seconds? White's whole speech about "he should have shot sooner, he should have shot more, he should have shot harder" was just being an armchair quarterback. there are a million things factoring into a fight.
If cain shot for a double instantly ala sonnen vs. silva and ate an uppercut and was put to sleep. He would look like a fool and it would have been "why didn't he set it up?"
If he stayed on the outside for a few minutes trying to tire JDS out and set up the takedown but caught a punch in an exchange and went to sleep it would be "why didn't he shoot? what was he waiting for"
It's very possible it was a fluke punch and Cain is 10 times the fighter JDS is, it's equally possible JDS is the next anderson silva and will knock Cain out every time.
All we know for sure is that the one time they did fight, JDS finished him in the first round