Although I do understand where you are coming from, there are a few things to point out.
First of all, Sakurai has not included any third parties for promotion. If you don't believe me, take a look at Bayonetta. Much like Rabbids, her game was in production at the time of the latest Smash (in this case, Smash 4). However, unlike the Rabbids, Bayonetta 2 was announced a little after development began on Smash 4, which was shortly after the release of Uprising. Despite this, though, Bayonetta was not included in the base game and when she was included, it was because of fan demand. It just so happened she had a recent game on Nintendo hardware.
Even if Ubisoft put 100% of their effort behind the Rabbids being included in Smash, Ubisoft isn't the one that's calling the shots. It's Sakurai and ALL parties have to agree on the choice. This applies to Rayman as much as it does to the Rabbids.
In the end, promotion has only been evident in first party inclusions such as Robin, Corrin, and Greninja. Third parties can push for their characters all they want, but Sakurai is the one who needs to see the potential in the fighter, and I don't see him starting third party promotions with Ultimate given the limited amount of newcomers we are getting.
That's my take, though.