That's getting a little too speculative...
All low priority characters are the ones in between R.O.B. and Snake within the data (why Snake and Sonic aren't counted is because the guests were last in the data on purpose), this is especially obvious with the fact that Jigglypuff, Mewtwo, Roy, and Dr. Mario are not with the other Veterans and are instead towards the end of the data after the higher priority Newcomers.
So regardless of the situation, Mewtwo, Roy, and Wolf were all low-priority.
Unless you're saying that Sakurai decided that Wolf was worth it more than the two, at which I can see that as a possibility for multiple reasons, such as his popularity, the fact he was easier to add (giving Wolf the same animations as Fox, then working from there, as compared to making the likes of Mewtwo from scratch), and the fact he was a Newcomer rather than a Veteran.