My view on things:
It is a stew of:
1. Marketing, could be better. Not as many people know about Windows Phone. It needs some average joe publicity!
1b. Public image, it could be better too, Microsoft doesn't have the best reputation and some discount Windows Phone simple because its fashionable for Microsoft to suck at things, especially among some techies. (See: Internet Explorer, the ridiculous subset of anti-Windows 8 arguments - also see: technically inclined acquaintance sees my Lumia and asks a simple question: "why...") It just doesn't have the hip image either.
1c. A subset of that is sales experience. Sales reps can be personally biased, misinformed, not trained, or paid by commission, usually not in Windows Phone's favor. that can seriously affect a consumer's purchase. (See: Countless threads regarding this)
1d. Did I mention marketing and awareness? A lot of people aren't aware of it.
2. App selection, it used to be a serious issue, and it still is an issue and I believe Windows Phone will have trouble shaking the "no apps" reputation without a "big guns" marketing campaign on the scale of Samsung. Oh, and getting more apps on the platform is still a work-in-progress. Still has a long way to go. My sister might have to forego a 925 because it lacked a certain medical application. (Haiku, or something like that.)
3. The smartphone market is notoriously hard to break into at this point. At least from what I see. Palm, Blackberry, and Microsoft, with all its resources, are still clawing for ground.
4. A slight sprinkle of specs might be it too, higher numbers can sell stuff.
And some other spices.
Season to taste.