I could speculate (and this is not my opinion, merely a possibility):
I don't think Mo likes players that are overly creative - or you could use the word "unpredictable"
Most of his better players are up and down the ice, pass, shoot, and hopefully hit the net. everyone once and a while these players try to run it through the defense but mainly they either pass or dump / chase on entry.
Ehlers is not that kind of player -
His zone entries are solid - best on the team but the up and down players he's lined up with have no idea where he's going or what he's going to do. In other words, he might be considered hard to play with from Mo's POV (possibly even the players POV). Mo might prefer a more conventional approach that includes less puck dragging, more passing, more to the net, more shooting, etc -
And I'm sticking to my disclaimer - this is purely speculation