>>40481561I don't even think its a case of "couldn't," I think Sakurai just preferred the opportunity to do a proper wrestler/grappler. Decidueye had plenty of potential but Incineroar had a clearer archetype and one that had never been fully realized in Smash before.
As someone who would've preferred Decidueye but didn't think either of them deserved to get into Ultimate, Incineroar is really well implemented and fun. Though I do wonder if Sakurai's choice was at odds with GameFreak's original expectations. Decidueye's sleek, manufactured-cool design definitely has some strong Greninja energy, and I feel like he might have been designed in part with the idea of making a Smash-ready Grass starter to join Greninja and Charizard. Of course, Sakurai just brought back Pokemon Trainer anyway. Who knows how many of the Incineroar perks (good movepool, strong hidden ability, prominence in the anime) were added on once he was selected for Smash. It all turned out OK in the end, at least.
Rowlet would've unironically been the best pick for Ultimate, due to both popularity and entertainment factor. Jigglypuff proves that NFE orbs can work, I would've loved to see what Sakurai could do with him. 