Same as WinPhones - except in quality (maybe)
I used to work for Motorola 2 years ago. A number of teams were made redundant then bcos management realised they were pissing in everyone's pint by having software for 13 different software platforms in active development (I kid you not). But most ppl never knew this, bcos all the phones had the same UIs. So long as they ran Java, they were all compatible as far as the user was concerned.
What a lot of folk (including apparently Mr Lettice) miss is that there was nothing wrong or even different about MS's approach. A lot of mobile manufacturers don't actually make the stuff themselves - they simply give a spec to someone else and say "make this". The UI is specified down to the level of individual menu items, what order they appear in, and what action is taken on each button press in each UI state. How the hardware responds to that is a problem for their contractor - the manufacturer doesn't care, so long as it does it. They often don't even care what hardware is used (processor, camera, etc.) so long as it has the right amount of RAM, megapixels on the camera, etc.
Where MS went wrong was simply in trying to make a mobile look exactly like a PC, forgetting that what works on a 1280x1024 screen with mouse and keyboard is unlikely to work on a 128x102 screen with cursor keys and numeric keypad. In other words, Windows phones failed bcos their UI spec sucked, pure and simple. It also didn't help that WinCE extracted a tithe from manufacturers which pushed per-unit prices up compared to in-house software platforms (or compared to freely-available platforms like Android), which makes the "ecosystem" less favourable. But ultimately, it all went wrong bcos MS didn't think properly about the UI.
Had they got that right, they'd almost certainly be a major player now. Instead, like in the Browser Wars, they produced something which didn't work and got crucified for it. But this time, unlike the Browser Wars, they didn't have any way of forcing their product onto the public.
With smart-phones today, WinCE could actually be a reasonable option. We've now got large enough screens and enough processing power to make it work. But MS screwed it up so badly the first time around that it's unlikely public opinion would ever let them have another go.