And then there's the odd model overlap of the 925, sold by both ATT and TMO. Yet very different in data bands and Wifi Calling (none on the ATT.)
Prepurchase of Nokia by MS, I think all the contracts were between the carriers and MS. Remember, WP is the operating system, just like Android is. Android does not contract with carriers.
In the old days when voice was voice and the only difference in carriers around the world was, "Which of four bands?" one phone model could literally cover the world, especially once four band became common. Data was voice tech based, 2G and 3G on GSM. Once we moved to data exclusive frequencies and technologies, we started to need carrier exclusive phones.
Of course, even before then, every carrier wanted their imprimatur on the phones. Boot screen, custom apps, etc.