This may just be me desperately grasping for straws, but an article about why Monster Hunter World is delayed for PC had a very interesting bit of info:
"There's extra work you need for PC as well. On console you use first party matchmaking, you just kind of plug into it, whether it’s PSN or Xbox Live, but on PC we’re going to have to work more to get our own matchmaking working."
Source:
http://www.pcgamer.com/monster-hunter-world-producer-explains-pc-delay/
If the game were only releasing on Steam, wouldn't that take out any need for them to have their own matchmaking? I feel like doing their own matchmaking would make more sense if they're planning on crossplay between Steam and GOG.