Microsoft themselves have stated that this year is pretty much a loss for Windows Mobile. They didn't really bother to market or sell their phones to ANY carriers except AT&T. The 950 and 950XL are just placeholder phones for now, keeping fans and developers happy until the new Surface phones come out next holiday.
Source:
Windows 10: Microsoft Doesn't Really Want to Sell Windows Phones in the U.S.
"We're refocusing our channel strategy, narrowing it in the short-term and planning for broader operator availability long-term," Microsoft says.
"While there was interest across the board from U.S. operators, currently we've made the decision to have AT&T carry the Lumia 950, and then sell both the Lumia 950 and Lumia 950 XL unlocked through our own channel in Microsoft stores," Microsoft's statement continues.
from T-Mobile's Smith: "I like Windows phone, but I don't think Microsoft does, otherwise they would treat it differently."