Director Zack Snyder only has love and respect for DC's movie and TV universes but maintains that the two should stay separate.
Crossovers are great because they bring fandoms together, boosting collective appreciation for characters and the series they come from. However, bringing together different characters can be troublesome because there runs the risk of making stories too complicated.
This is what Snyder wants to avoid.
"As opposed to trying to shoehorn all of these storylines into a single universe, we let the characters exist in multiverse, and therefore it's a lot more fun and a lot more value for the audience," said the "Batman v Superman: Dawn of Justice" and "Justice League" director.
There's more fun and value in that the audience can see their favorite characters in different adventures, made possible to occur at the same time because different universes exist. This results in a lot of fun too for the people behind the scenes like Snyder because they have the leeway to explore different directions for the characters.
At the moment, the property most affected directly by the separate movie and TV universes DC has is "The Flash." Grant Gustin has become the face of Barry Allen for many TV viewers today, appearing as the Scarlet Speedster on The CW's show, so it created a bit of friction when Ezra Miller was cast to be Flash on the big screen. Miller has been dealing with the backlash graciously but it got out of a hand at a certain point that Gustin himself showed his support for the actor.
One argument for allowing movie and TV universes to interact, at least in the case of "The Flash," is that it could result in an audience boost. "The Flash" is no stranger to embracing multiverses so it could pull it off.
Crossovers have also been done in "Arrow" involving characters from "Suicide Squad" but they have not been allowed to flourish, possibly because DC wants the audience to focus on the movie version of the characters.
