I have a few guesses...
1) Marketing. Like everyone else said. I don't need to elaborate too much here.
2) App ecosystem. Lets be honest, while my near year with windows phone has seen a dramatic increase in apps, we still lack a good amount. some banking apps, BBM, candy crush, etc. Believe it or not, I know someone who got rid of their 1020 because they couldn't play candy crush...
3) Late to the game. If WP 8/8.1 had debuted around the time Android did, i think we'd see a much different playing field. just speculation on that one, really.
4) People don't like change. Take me for example. i used a blackberry tour from the day it came out until...I THINK July last year? Then I got a galaxy note. it was such a hard change, because lets be honest...blackberry was awesome. Then going from Android to WP was ANOTHER huge change for me. granted, a change for the better...but a change regardless.
those are my thoughts, anyway.