The iPhone is a HSPA GSM phone. Sprint and Verizon are CDMA networks.
You will never see the iPhone on their networks unless they build one specifically for CDMA, doubtful, or if Verizon and/or Sprint build a GSM/HSPA network.
Rogers and Fido had an exclusive with Apple strictly because they were the only GSM providers in Canada. Bell and Telus went out and built a HSPA GSM network and now also carry the iPhone.
Almost all countries in the world, except for the US, South Korea, and parts of Iraq now run GSM (Global System for Mobile Communications: originally from Groupe Spécial Mobile).
GSM HSPA+ is faster than anything available right now, including Sprint's 4G.
HSPA also allows you to talk on the phone while still using data, CDMA does not.
You can bring your phone anywhere in the world and your number follows you, provided it's not one of the 3 mentioned countries.
You do the math, one version for basically the entire world, or 3 for the US (CDMA 4G, CDMA LTE, and GSM HSPA) and one for the rest of the world.