Because Sprint the only one offering themselves up to MVNO's. It's a symptom of a failing company and exactly why you don't see ATT or VZ whoring their network (especially their LTE services) to every Joe MVNO.
Thants because most other providers are not stupid enough to take them onboard. But what happened to ESPN, Brighthouse, Roadrunner, Commcast? Now, they sell a lot to republic wireless and VM (and a few others )