If we're talking first-parties I'm still gonna push Isaac. First-party Banjo, I tell ya.
Going more in-depth, there's three reasons: The Ballot, the AT reveal and the negative reactions to it, and the new Golden Sun trademark. Isaac must've scored pretty high on the Smash Ballot; He was pretty darn high in various consolidation polls, including
this one, which is apparently the largest of the consolidation polls. Not sure if he was
exactly that high, but he must've been done pretty well. Plus, he got the Assist Trophy back, as well as a pretty powerful Spirit (The Spirit means less than the AT, of course, but they're scaled more on popularity than anything; For example, Trevor is a four-start while Julius is a mere two-star). Seeing as there's nothing Golden Sun in the works, it's most likely fan support.
Secondly, the AT. Part of it was everyone thinking it was his time (The GS item, the Rathalos pic, the Grinch leak, and general popularity), but I hear his snowglobing was met with very negative reception. I'd imagine Nintendo would be aware of such backlash (I think I heard somewhere that they actually did, but I'm not sure it's from and I don't want to spread misinfo if it's incorrect or from a dodgy source).
On that note, the eligibility of ATs. I think you know what my stance on ATs are by now. They weren't chosen with a second wave of DLC in mind, Spring Man was used in the ARMS guessing game and prominently featured in Min Min's trailer, Sakurai is fine with multiple Bomberman Miis near-identical to the AT running around, Nintendo most likely wouldn't care about something as arbitrary as that, yadda yadda. The only thing holding ATs back are their own merits, which I'm going over now anyways.
Lastly, the new Golden Sun trademark. I would hope everyone knows about it by now. A new, NON-RENEWAL trademark was filed in 2018, turned down twice, and finally passed this March. If it's just a port or emulation like with the recent Shadow Dragon port, it would use the original trademark and not a new one. If it failed twice before failing, it sounds like Nintendo wouldn't want to give up on it easily. At the very least, it sounds like Nintendo still cares about the series, and not just in the "Renew the IP that I'll never use" kind of way.
...Is he a lock? Of course not. I'm just pretty optimistic for him.
Though I'm optimistic about like 80% of everyone I want so it's not like it's saying much Probably didn't need to write that much, but eh. Suppose I was just in a writing mood this morning