there was so many kind of apps you simply couldn't write for windows phone 7 even that it was not funny.
MS/Nokia was dishing out cash and free devices and lunches to everyone, but their dev relations to questions "when will this or that be added to the api" resulted in "you don't need it". which was puzzling since making a decent version of the app depended on having that.
anyways, windows phone sucked big time. easy to develop for but so very much limited and not extensible at all - wp7 was so bad that by 2003 standards it would have been called a feature phone, not a smartphone(no real full multitasking and stuff that used to be the separator between a smartphone and a feature phone back in the day).
wp7 was featurewise equivalent to j2me phones and everything was very, very betaish despite being super simplified.
anyways, it all traces back to ZUNE - all of the crappy decisions and failures MS has done in the past 10 years goes back to the ZUNE. Wp was just a rehash of ZUNE shell, rushed. it's so simplified because thats all they had! and then they tried to cover it as being great because it does nothing.
a smartphone needs to have decent multitasking. win ce had it.
the whole problem was ditching the old stuff and replacing it with new stuff that wasn't ready. sure, one could live with wp. but why bother when there's android.
and being forced to use win8(and now win10) for development isn't exactly a plus either, especially when the dev env doesn't really depend on any win8/10 features.. oh well at least they were giving those license out free nilly willy too.
anyways, windows phone was never relevant in any market. the only place where it was slightly relevant was Finland due to loads of organizations sticking with Nokia's as their organization provided phones - and because nokia and ms were just giving cash to publish stuff.
And Microsoft has just about given up on it as well. It's now just this thing they have.
If you want a real explanation it's simply that MS board consists of idiots. how can so well paid people be idiots? well look at what they have done and bought in the past 10 years and how they have ruined their core product. it would have been better to do absolutely nothing. not buy nokia, not linkedin, not publish win8rt, not publish win8.
they could have bought ARM holdings with the cash they want to pay for linkedin btw. that should put things into perspective how much they overvalued it.