iOS is considered to be the simple/accessible/reliable option by default. Android is the geeky alternative that is intended to do be everything to everyone. It's hard to define a marketable identity between those two extremes, which is the main reason WP is hard to sell. It lacks an identity. Those conversing in these forums admire WP for different reasons, but ask anyone on the street what WP stands for, what its main advantages are, or for whom WP is best suited, and all you'll get are blank stares. More often than not, all people will have heard is that "it has no apps" :-/ (which is no longer true)
Having to explain to people that Windows Phone is not at all the same as Windows is another thing working against it. With the exception of the kernel, which is entirely inconsequential for anything related to the end user experience, the two OS' are as different as they could possibly be. Misconfiguring a WP device or having it slow down, just by installing poorly written apps or a combination of conflicting apps is pretty much impossible. In terms of security, WP now rates even higher than Blackberry, but that is not how anything "Windows" related is perceived.
Finally, just sticking with the popular choice, or preferring a new version of what you already know, rather than having to learn something new, is just human nature. This is also a huge issue working against WP. If WP ever wants to shake people out of their habits, then it must offer something truly desirable that is instantly and recognizably different/better. Most people won't consider WP before that happens. Until then they will stick with the OS they know.
Despite all that, there are many people for whom a WP device is the best match, but you almost have to be a regular at WPC to understand who fits that profile.