Crystal Dynamics' Studio Head Scot Amos has spoken about the decision (reported via NewsWeek), and it doesn't make for great reading for Xbox or PC fans.
Crystal Dynamics Studio Head Explains Spider-Man Exclusivity
According to Amos, the addition of Spider-Man was only possible due to Marvel's relationship with Sony. After all, 2018's Marvel's Spider-Man is the best-selling superhero game to date, and Sony still holds all the cards when it comes to the iconic character's movie appearances, too.
“Because of Sony’s unique relationship with Marvel and PlayStation, specifically, that gives us an affordance with Spider-Man that we wouldn’t have otherwise. So that’s something unique because of that relationship” Amos explained at a recent press event.
“We happen to be ones who can execute and deliver when it comes down to choices of where and what Spider-Man can be, that’s a relationship question that you know PlayStation absolutely has the rights to that as you guys know with Sony’s [movie rights] ownership there and Marvel with Sony saying, ‘hey, this is something we can do, this is something we can do on this platform’ and as creators, we have this opportunity that we can make something unique and fun and awesome." he explained.
It's all a lot of word-soup, in truth, but when pressed about how Xbox and PC gamers could play as Spider-Man, Amos explained that they "have the option to do so on PlayStation".