Mario Mix characters- According to KPG65 working with Mario Mix owners was never fun, mainly due to lack of communication with the owners.

Wario Apparition- Lack of moveset potential

Mario the Immortal- Same reason as Mr. Virtual being deconfirmed

Stanley- Likely due to his hostility being mixed as well as lack of moveset potential

The Hunter- Drama surrounding one of their creator's characters (The character itself isn't controversial but the archetype/character type is)

MX was deconfirmed for a multitude of reasons, firstly the developers don't care about him because he may be too popular, 2 he's already in Pillar Chase 2, and 3, the creator of MX doesn't want him in games outside of the ones he's already in.