From what I see:
1. They wanted Damian's team to be perceived as the premiere team between two books but readership picked Titans instead and whatever they had going for the book got overshadowed by the unexpected success of Titans. Both Abnett and Booth said they didn't expect the book to be successful which can be taken as a proof of why Titans doesn't have a solid direction after a year. With this merge, Titans readers will have to read about Damian's TT, the team they REALLY want to push.
2. They are opening a slot for the upcoming Young Justice book.
Like others have said, I'm mostly worried about Wally and characters who will never get a spotlight like Lilith and Garth. Titans had something going for them and now, a potential merge will sideline them again (why have Omen when you have Raven?) I really want to read about Wally and an expansion on the roster will only diminish the spotlight he gets. I hope they know and understand that this sort of move requires Wally to be moved to his own book.