Because the devs/publishers aren't the ones paying to get their games onto Gamepass. Also, we know by demographic,tendancies and sales statistics that JRPGs are more popular and viable on Sony's platform than Microsoft which is why seeing a port of Octopath Traveler on Xbox first before the PS4/PS5 definitely make it hugely possible that Microsoft had their money involved into this.
Anywoo, the same will happen to other SE Switch exclusives in a matter of time after the exclusive deal has waved out. Though can't say if it'll be only on PC or every console thus far since we only have Octopath Traveler for example here.
I mean, both the individual Xbox release and the PlayStation versions could be announced in the coming hours. Hard to say, though.
My bet with The_Liquid_Laser: I think the Switch won't surpass the PS2 as the best selling system of all time. If it does, I'll play a game of a list that The_Liquid_Laser will provide, I will have to play it for 50 hours or complete it, whatever comes first.