As people have already mentioned it comes down to a few things.
1) brand. Apple and Samsung have huge draw to them, Microsoft is still seen as this relic that needs to die off. Not only that, but Windows phone is seen as a very cheap brand. Not a high end brand. Most used Windows phone last quarter: Lumia 520. An old cheap as chips phone that you can probably get for about ?20 off contract.
2) apps. While I'm set for apps, most people aren't on Windows phone, and this makes it a no go.
3) OS. People want an OS that works solid, that has all the features to support the apps. It should just work, and while I do like W10, even I have to admit it's a little rough around the edges and this puts many people off.
4) design. I'm not gonna lie, the 950/XL looks bland, boring, and uninspired. If the phone looks like a turd, the general consumer won't even give the specs a chance, if they even care about them in the first place.
5) sales reps. The reps promote what they use, they're firmly on iPhone/Android. Plus since the Lumia 950/XL supposedly has higher return rates than the others, why risk selling it when a return could see you get fired from your job?
6) marketing. Where are the adverts promoting what this phone can do? How are they promoting this as the best thing since sliced bread? Why not put down money promoting Continuum?
All of these create the impression that the phone isn't worth the asking price, rightly or wrongly. In the eyes of the general customer I'd say the phone is worth about ?0.01 - no I'm serious. That's how bad the Windows phone rep is seen. Case in point, I was showcasing Continuum at work yesterday, they were like "Cool, but can you get it on iPhone?" They weren't going to swap to Windows phone just for Continuum. That being said it's a feature I use a lot.