WP isn't sold using the same model as iOS is. The following discussion applies mainly to the North American markets; the rest of the world tends to be different.
Apple makes iPhones and the software for them, so Apple is the sole provider of the OS, the firmware, and the hardware. Apple also has clout with the carriers, since no carrier is going to haggle over Apple's distribution Terms and Conditions - any haggling will be about how much money per device the carrier gets. As single source OEM of a very desirable and profitable product line, Apple is in the driver's seat.
Microsoft's OS group makes WP, and 'sells' it (gratis, now) to OEMs. Yes, the largest WP OEM now belongs to Microsoft, but that is a different division. It still has to be treated legally as an OEM to avoid contract and perception problems with other OEMs and to avoid potential antitrust issues with various governments (US, EU, etc.).
The OEMs, in turn, provide the OS, firmware, and hardware to the carriers. With WP OEMs, as with most Android OEMs (the main exception being Samsung), it's the carriers that have the clout, not the OEMs. The carriers want to keep their support costs low on non-Apple, non-Samsung phones, so they insist on approving OS and firmware releases on the phones they sell. Remember, most phone users in North America don't contact the manufacturer or the software developer when they have a problem, nor do they cast about on the internet except as a last resort - they take the phone back where they got it, which is more often than not, their carrier.
Microsoft the OS maker has come up with a way to bypass the carriers - the DP program. They can do this because they don't sell the OS to the carriers, and the OEMs are willing to allow it. Of course, the carriers and OEMs can choose to void the warranties on any phones running it, although so far, none of them has. The OEMs, however, can't bypass the carriers as easily, since they're the ones that actually have contracts with the carriers. Also, firmware is much more likely to affect basic functions like making calls and sending SMS texts, things that people are likely to blame on the carrier if they don't work, so the carriers are a lot less likely to look kindly on the OEMs trying to bypass their testing.
Bottom line: Until a WP OEM has the kind of clout with the carriers that Apple and Samsung currently have, or until North Americans shift more to the world model of buying unlocked phones from vendors other than their carriers, the situation is unlikely to change dramatically.