I mean, Antonio Del Rio voices in Uprising and SSBU... is there something I'm still missing here?
Generally how it works is that union actors get paid more and have the union backing them up if unfair treatment happens. Non-union actors typically don't get paid as much because they don't have a union fighting for higher wages. A union actor can usually take a non-union role, though, if they go uncredited, which a lot of the time is a big thing to ask.
Nintendo is notoriously cheap and rarely ever uses unions for games. Off the top of my head, Fire Emblem Fates, Code Name: S.T.E.A.M., and Kid Icarus Uprising were all union projects, so they got more prolific voice talent. But in later non-union stuff, they either don't reprise their roles, or go uncredited like Antony del Rio.
Ideally everyone would be a union actor in a perfect world, as unions are objectively good and provide job security and will fight for their actors' rights, but the reality is a lot of companies are also really cheap and would rather use non-union talent.