It likely has to do with the movies, the forthcoming Supergirl, Gotham, Arrow and Flash all being produced by and having different people with the creative control.
Arrow and The Flash can more easily share the same universe because they're both on the same network. But in the end you've got:
The CW: Arrow + The Flash
FOX : Gotham
CBS : Supergirl
TNT : Titans (possibly)
Who wants to coordinate that? What do you mean you'll have to delay beginning of the crossover to next week because of an NBA game? What am I going to do with just the END of the crossover until then? We have already advertised for this, you know? No you can't replace Barry Allen with Wally West, we have already shot Barry for an episode that will air in January. No, her costume needs to stay as it was in the document from last May. And her name is spelled K-A-R-A. So... You realize that would make Robin older than Batman, right?
At least it seems the movies are all made by WB?