I'll tell you why, because there is literally no other sane option.
All the carriers know that consumers want an individual plan that goes like this:
500 minutes, unlimited texts, unlimited (5gb+) data all for around 50ish dollars a month.
Sprint offers this, no one else even comes close.
I am on the Sero Premium plan with Sprint, and believe me I would LOVE to be on TMobile or AT&T due to flexibility with GSM phones as well as their much more mature networks. The ONLY plan that even interested me at all was the Tmobile prepaid walmart plan for $30 a month, but that only gives 100 minutes, unlimited texts and 5gb+ of HSPA+. If I could make 100 minutes a month work, it would be great, but that is just too small and I hear very conflicting reports on using a SIP solution to take calls over a data connection.
So what options are there for people? To pay literally double what I am paying now for faster data speeds?