What presedent is there for them to go exclusive for only a single region?
Well there's Destiny to show for it in which Sony got exclusive publishing rights solely for Japan ...
Even if Capcom did get dev kits early and wanted to port MHW to the Switch badly there's still no guarantee that the system can run the same game ... (that's the most probable cause so Capcom would've needed to make MH5 for Switch regardless)
That is true but thats a bit harder to ascertain. When Desiny released it was the last year SE had publishing duties for CoD in Japan.
The following year, publishing duties were given to both Sony and MS. Both had to publish, market, distribute and localise the games themselves. MS coud have easily just decided they werent interested in the costs to localise the entire script, publish and to distribute it. Keep in mind with Destiny being an ever evolving game, they would have to be comitted in the long run and continue to localise update and still sustain costs for whatever their publishing duties involved.
Its also worth noting that from what I can gather, MS is not publishing CoD this year either. With only 81k X1s sold, its no surprise. So Destiny 2 is not being published by them either since those costs would be way more.
tldr, yeah Destiny is exlusive to Sony but that can potentially be because MS were not interested in the costs involved in localisation, marketting, publishing and distribution, since they did publish 2 years of CoD even after the IP was pushed heavily with Sony.