For one this is uncharted territory in being DLC. Before, we've received new characters, but nothing ever matched up to coincide with other releases. The DLC we've received thus far didn't match with anything because there was nothing there. There was no opportunity to fill. Now there is. I'd say the same thing if we had a Donkey Kong game that was within the proper time frame. Hell, there may be one because I haven't checked. Regardless, my argument stands that this hasn't been a trend because there really hasn't been much of an opportunity. Though, now we've starting to see it with the Super Mario Maker stage. Why not fill a ballot option with Wolf or Krystal during what would be a prime opportunity?
We've seen Mewtwo, but that was a promise way before the game released. His purpose was to introduce us to the idea of DLC while satiating the want for that character for ages. Next, we had Lucas, Roy, and Ryu. Lucas had no real game to coincide and was once again just a fan-service returning veteran. Roy I feel was the same purpose. Ryu on the other hand, why was he there? We know there was little demand for the character. So, my hypothesis would be that the character was added as per agreement with Namco. It could have also been a slight promotion for Street Fighter V which is to release in 2016, but that seems a bit far off. I believe that character was also implemented to show that they are willing to broaden their view as far as what characters can be added from the ballot.
As far as I see it, this is one of their prime moments to use the DLC for promotional gain. This certainly wouldn't be the first time a gaming company has decided to introduce something to us at a pivotal time.