Comment by eloisant
To be honest there were a lot of mobile OSes at the time supporting multitasking, like Windows CE, because they were desktop OS (Linux for Maemo, Windows for CE) with little adaptation for mobile. That meant performances and battery life were not great.
That's why those OS were mostly used by geeks and power users, and "regular" users were using feature phones that "just work".
One of the strength of iOS and Android were to create a completely different userspace that what we had in desktop OS, more adapted to mobile. They combined the "just works" aspect of feature phones with the power of smartphones.
Windows CE is quite internally different than Windows NT. It still does support multitasking, but kernel version 5 (which was on all the CE devices of the late 2000's/early 2010's) had a maximum of 32 processes. It was a platform specifically for embedded use, though the GUI was styled to resemble Windows OSes at the time and of course numerous Microsoft things were ported over.
Windows Phone 7 moved to CE 6.0, then Windows Phone 8 to 10 were NT based.
Wikipedia says Windows Phone 8 was released October 29. 2012, which is around the time the ARM-based Surface RT was also released. A significant event for Windows NT to be on an architecture other than x86.