I would guess that this move is because because their software runs like a bag of hammers falling down a flight of stairs and the last thing they want to deal w

I would guess that this move is because because their software runs like a bag of hammers falling down a flight of stairs and the last thing they want to deal with are your support calls about why their products dont work right. Don't get me wrong, they can work smoothly- just it takes an order of magnitude more work than you would expect / they would admit. To be slightly more charitable, this is a sensible move as at least this way Atlassian can control the end user experience and (perhaps!) have it not be utter trash. Far easier for them to manage this way.