WE also haven't seen a successful or any Tim anywhere outside of comics. Arkham Tim wasn't Tim. DC has never not once in his 30 year life been interested in featuring Tim outside of comics that one time in BTAS 100% Jason.
If they won't give him a chance in animation why would they in life action?
Plus we've already seen a random helping Batman in TDKR and it didn't set audiences on fire why do a repeat?
But most importantly like I said before it comes down to who they want not who is easier. Let's say that Damian is indeed harder but the payoff is far greater so there's.
I'm not sure if there will be a robin in the DCEU but if there is I don't see them going radom guy when it could be the son of Batman.