The wireless carriers have the final say on which phones they stock and promote, not the manufacturer. There most likely weren't enough sales of Windows Phone devices to make it worthwhile for them to bother trying.
Windows Phones had a different UI that meant the sales rep had to spend time showing how it worked, and even if he/she did make the sale, he faced the high risk of a Lumia getting returned because the customer found out some key app wasn't available on Windows Phone but was easily available on iOS and Android (their bank, or airline, Snapchat, and let's not forget Instagram wasn't there for a while)
(source: I'm a former Nokia employee)