Probably the agreement between Apple and the US carriers. It's been like this since the days of iPhone 4. Prior to that, Apple didn't even sell SIM free phones in the US. Luckily with LTE, Verizon iPhones are unlocked out of the box, so those looking for "SIM free" iPhone from day 1 can just get the Verizon version.What was the reason for NOT allowing SIM free iphones? Could buy them from day one in Canada.