That's why. I won't dispute your good experiences with Sprint in your area. But a lot of people here got into Sprint a few years ago when they were offering unlimited data for cheaper than Verizon or AT&T. Unfortunately, Sprint's network is bad. It's not bad EVERYWHERE, just in many places. For many people, Sprint 3G is so slow that it's unusable. You have an experience like that where you're paying $70+ per month for a shitty unusable smartphone, and you're going to jump ship and never look back.
That's what I did, and it's what many others did.
I do like Republic Wireless' plan structure. I like that they offer a no-data tier and let you pay extra for 4G if you want. What I don't like is how you can't bring your own device. That alone is enough for me to want to avoid them. It locks you in to using Republic Wireless - have a bad experience and want to switch? Too bad, looks like you'll have to buy new phones. There's no ETF but you're still losing money.
Now I know that there isn't a great deal of mobility from one carrier to another in the US because of different carriers using different frequencies and technologies. But you can still switch between different providers on the same network, with some crossing over between AT&T and T-Mobile if your device is unlocked. I'm currently on Platinumtel (T-Mobile network) but I could cancel and go to Straight Talk or T-Mobile Prepaid anytime.
Republic uses the Sprint network but you can't even switch your Republic phone over to Ting or anywhere else. Just too much of a restriction for me.