No idea. I suspect the bare minimum of testing was done at least, and it's entirely possible that the machines used in testing don't suffer from the same problem due to some sort of...I don't know, specific internal setup they have for developing and testing things.
As for why, as I understand it Microsoft is the primary reason the game was brought over, and they also host the servers on Microsoft Azure. That's why it was an Xbox exclusive, and then exclusive to the MS store on PC. They did say they're bringing it to "other PC platforms" though, but have not yet mentioned a date. I suggest waiting on that if you enjoyed what little of the game you were able to play.
It's certainly an extremely disappointing first impression of an otherwise great game.