What? They didn't go the extra mile and provide custom moves? Seriously? This shouldn't have taken so long to bring out then. Seriously...it is basically just a slightly balanced and modified Melee character with a new HD model.
Mewtwo wasn't the only DLC character they've been working on. Like the roster, all the DLC characters except the winner of the fan request would have been decided on long before their development even started. Even though Mewtwo came out first, they've been working the other ones this whole time as well.
Plus Mewtwo in his current form is already messing with the games data and causing glitches. They already found evidence of cut customs moves from Mewtwo as well which wouldn't surprise me if the DLC characters originally were going to get custom moves but trying to implement them was causing too many glitches so they decided it was easier to just not give them any.
The thing about these DLC characters is there's no data for any of them in the original versions of the games. So suddenly just adding them into the main game for something as technical as Smash Bros isn't as easy as copy and pasting.