Where have Activision said they wanted him in? I heard two Acti guys say on a stream that Crash in Smash would be cool pre-Ultimate reveal Post-March Switch reveal.
Crash's revival popularity, in the US at least, coincided with Joker's and Eleven's in 2017, and assuming the DLC was decided sometime up to early November in 2018, they could have given him a look even if it took until like November for N Sane to release on Switch in Japan. Activision and Nintendo previously worked together for Skylanders, and in 2018, Activision managed to successfully release N Same on the Switch from an employee playing around with it, and announce a new game coming to multiple platforms including Switch simultaneously. Crash seemed to keep recognition in Japan, what with the Famitsu 30th anniversary art and his song placement in an upcoming Japanese Game Music event. He could totally be another pair with a more Japanese rep, maybe that's where Nintendo's going with this. Maybe they'd like to see a more general (not already Nintendo associated?) Western character as well? Does he have much competition that's also beloved in Japan and not Microsoft-tied?
Sure like Banjo he's also a Western 3rd party, goofy and cartoony animal mascot platformer rep, but if we can get two JRPG animesque sword and Spellcaster men with extreme Japanese popularity, I don't see why we couldn't also get that.
I don't know if Japan specifically had as much of a "Mario Vs Sonic Vs Crash" thing going on to factor into the inclusion, but Sonic's rivalry seemed to be important at least