Biggest factor: ROS. Berdych has no problem returning Gael's 2nd serve. In the same time, Monfils isn't returning great, and he needs to return great against Berdych. That's why Nole breaks him to pieces.
yes in many match-up specificities the ability of one player to read and return an opponent's serve is a big key because it's a surprisingly very individual thing. And here it's one key inded as Monfils said himself after the match : Berdych returns Monfils' serve very well (which, according to Gilles Simon's own words, is a actually a tough serve to read), and Monfils has difficulty to return Berdych's serve.
Besides the bounce of Monfils' ball, quite high but not too much, seems to fit Berdych very well, whereas he doesn't like Simon's lower balls or Nadal's high spin. And Berdych is very much a player of rhythm : if he's in a good rhythm against a specific player he's very tough to beat, and Monfils' ball is clearly good for his rhythm.
And Monfils is not good at varying angles and directions then can't break Berdych's rhythm through that.
Finally, as Simon said, Monfils doesn't have a comfort zone against Berdych where he knows he can play when he's in a bad position like playing to Federer's or Dimitrov's backhand : that makes him feel comtortable against them according to Simon's words whereas he doesn't feel that against Berdych who can accelerate his backhand very much.
Finally there's of course the mental aspect which comes both from Monfils' previous tough losses, and also partly from the fact, that Berdych, according to Monfils' own words, is his favorite player to watch.