Its pretty simple equation in the US - cheaper devices.

If you buy a unsubsidized device you sill pay the same $X for the plan each month as you would with buying the subsidized phone.

The only "extra" you get buying unsubsidized is the no-contract on your plan.
But if I was to stick with T-Mobile for 2 years or more as such (like I have with AT&T), then the "no-contract" deal does not have the same value proposition in terms of money.

In that sense the phone is actually cheaper unsubsidized in terms of raw money that I have to pony up over the long term.
To most US consumers the freedom of having no contract is NOT a value proposition at all.

Sad but true.