Xbox, Playstation, and Nintendo are primarily gaming platforms, and Epic is a major player in the gaming space. I'd guess they're much more accommodating towards Epic than Google or Apple who seem to treat gaming as more of an afterthought.

Also of note, you have to have Xbox gold to play Fortnite online. This is pure speculation, but that means that Microsoft could take a 0% cut on in-game purchases and still make a profit by distributing Fortnite, and that could be enough of an incentive for Nintendo and Sony to play ball. Sony especially cannot afford Epic leaving their platform in favor of Xbox, which is likely the same reason they eventually caved on cross-platform support.