This list appeared on Google+ after Motorola compiled their list of the Moto 360 Top 10. This list was compiled using the +1 feature on all the submissions on the contest page. Essentially, this is the voice of the people, not the voice of the corporation.
Seriously, the only reason I can think of to not have T-Mobile is of you live in an area with shoddy T-Mobile reception or slow data speeds. Evey time I hear about how InsertCarrier is changing their pricing I laugh all the way to the local T-Mobile store.