In 2013, a reboot of the Superman franchise, Man of Steel, was set to lay the groundwork for future DC films.[14] The film contained references to other characters in the DC Universe, so if it were a success, it could launch a shared universe.[15][16] Days before Man of Steel's release in June, it was reported that director Zack Snyder and screenwriter David S. Goyer would be returning for a sequel, which was being fast-tracked by Warner Bros.[17] In July, at San Diego Comic-Con, it was revealed that the sequel would include Batman in a main role, thus establishing the shared universe.[18][19] In October 2014, Warner Bros. announced a slate of ten DC films.[20]

https://en.wikipedia.org/wiki/DC_Extended_Universe

Nobody talked about DCU before MoS, I don't remember anyone even mentioning it, only reason why MoS is part of DCU is because it made a bunch of money, WB stared thinking about more movies only after MoS, they quickly threw in other heroes and announced bunch of other movies.