And Bill Gate blame it on Anti-Trust and monopoly lawsuit against them.
It was. I used it. They ported the Win95 desktop (right down to the task bar and start button) to WinCE and a screen the size of a couple of thumbnails. The GUI was horrid, but evolved rapidly. At the end WinPhone pretty, very uniform even across 3rd party apps, faster, and used less power than either iOS or Android. I'd say they executed WinPhone GUI framework extraordinarily well.
WinCE on the other hand never worked reliably on any device I came across. They stuck with it till well after iOS and Android had left them in the dust, and then transition to NT made the Python3 transition look well done - you had to bin your old device. WinCE blue screened more than Win95; a remarkable achievement. This was true for Microsoft's but doubly so for OEM's as MS made getting the source as exercise in pain, so debugging why an interrupt disappeared up it's own arse was basically impossible - you spent 1/2 your time reverse assembling MS's machine code so you could decode the stack.
I suspect the fate of WinPhone can be put down to that. iOS and Android went with robust, mature base OS's (BSD and Linux), that came will all the batteries required in the box and had all their rough edges knocked off by being ported to every platform under the sun, where Microsoft stuck with their home baked embedded OS, WinCE, which felt like a multitasking version of MSDOS / CPM, and they had to develop everything themselves. By the time they fixed that mistake, it was too late.
I'd say open source crushing all comers in the new mobile space is when it came of age, which is to say everyone from C levels down decided it was here to stay. That had nothing to do with technological superiority. When the largest software corporation on the planet pitted their embedded solution against open source, and was wiped from the floor, even a banker could see what the new lay of the land was.
But that still really would be the antitrust suits themselves since 1) MS got themselves into that mess in the first place and 2) Bill Gates shouldn't have let it distract him so much from potentially vital projects. 3) employee accounts of the internal politics at the time pretty much meant that most new products were doomed to fail from the outset if management on the Windows teams thought they were even the least bit of threat to them.
Gates even admitted he messed it up and lost $400bn with mobile operating system. https://youtu.be/W5g4sPi1wd4?t=709
There is a different story/side to this statement.
Fixed that for you.