Retailers couldn't sell what the carriers didn't want on their networks. The carriers had momentum from consumer demand to keep selling iPhones. The carriers were given a lot of the "keys to the car" by Android and carriers were really happy with the ability to modify Android and/or micro-manage it, so they had a lot of incentive to focus on Android.

In the US, Windows Phone tried for the "iPhone experience", which made carriers unhappy and less likely to want to sell it, which ultimately left it the case in the US at a point where only one US carrier at a time was even "exclusively" selling the latest Windows Phone hardware, and only through its dedicated retailers. It took too long for Microsoft to also realize that part of the iPhone plan in the first place was direct to consumer sales and pressuring the phone carriers to provide SIMs rather than making "exclusive" hardware deals with carriers and hoping other carriers would try to compete for buying your hardware as well.