Just FYI - Your question insults the very people to whom you are supposedly directing this question, and probably should have been locked by the mods, but I'll answer it anyway.
Originally Posted by vp710
The answer is none of the above. The answer is Economics.
Your question is actually a conflation of several distinct issues, which may be why you were unable to reason your way through to the answer. There isn't really a "Windows 8 platform" for developers in any meaningful (non-marketing) sense. There are three development platforms:
a) The traditional Windows platform as used from Windows NT 3.5 through the latest-and-greatest iteration Windows 8.
b) The Windows phone platform that was introduced in WP7.
c) The WinRT platform that was introduced in Windows 8, and used on ARM tablets running Windows RT.
The fact that all of these have the word "Windows" in them is marketing. From a developers perpective these are all different platforms, as different as OSX, Linux, iOS, and Android. This in important to keep in mind, because the question for a developer is not "why not develop for WP8 or WinRT", the question is "which X do I develop for". WP is competing with Android and iOS for developer attention, just as WinRT is competing with Android and iOS, and Win32 is competing with Linux and OSX.
Platform (a) Win32 is indeed a great platform to target. It is extremely capable, with hundreds of millions of potential customers, and is very mature, meaning it is well-documented, well-understood, reliable, and any development shop has a lot of code tied to that platform.
Platform (b) WP7/8 is a much less attractive proposition, which pains me because I've been a fan of it (Focus, Lumia 900, now Lumia 920). There are only a few million potential customers, the platform has had very little growth in the last few years and while this may well improve with WP8, there is no guarantee of this. The platform is still very immature and feature-poor compared with platform (a). The API is based on Silverlight, which has been deprecated elsewhere by Microsoft - at this point any WP7/WP8 application based on that API is living under the sword of Damocles. Meanwhile there are two much more economically attractive alternatives in iOS and Android. Any established development shop looking to get into mobile is likely getting into mobile because their existing customers are clamoring for Android and iOS apps, not WP7/WP8 apps. I am the senior architect at my company, which involves doing things like evaluate and recommend strategic development platforms. I have recommended against developing for windows phone for the forseeable future, in preference to iOS. I enjoy using my windows phone (have converted several coworkers to WP - including the company president) and dearly hope to see it succeed, but it simply does not make economic sense for us to put significant development resources towards that platform.
Platform (c) WinRT is an interesting one. It is an *extremely* immature development platform; the documentation is almost non-existent compared with its competitors, it is buggy as heck, with many missing features compared with platform (a), and shows many signs of being a tremendous rush job. There are huge swathes of applications that simply cannot be implemented in it, as Microsoft will admit when pressed (see Office and Dev Studio). Also, right now there are only a few million potential customers, but that pool is increasing very rapidly and will certainly reach the hundreds of millions soon. However, almost all of those customers can also run applications for platform (a) on that same machine. So they're a much less attractive target - if they already have your desktop app, how willing are they to buy your WinRT app as well? If somebody doesn't own your Win32 app even though it's been around for some time, what makes you think that they'll suddenly decide to buy the WinRT app? For the short term, the real market for winRT apps are the guys with Surface RT tablets, because they cannot run any applications written for the other platforms (unless you're working for Microsoft, in which case you get a special dispensation). But that is a very small market. Again, it doesn't make sense to target this platform when our customers are rapidly adopting iPads for their tablet needs. It may eventually make sense to target this platform if it gains traction among users, and once the WinRT platform matures a bit. I'm keeping an eye on it, but really don't expect it to become feasible for the sort of applications we develop until at least Windows 9.
I'm aware that WP and WinRT are stuck in a chicken-and-egg problem. But from a developer's standpoint it just does not make economic sense for many of us. Other developers may well evaluate these platforms and come to a quite different conclusion - some developers are writing WP and WinRT apps after all. WinRT and WP are better suited for certain types of apps, and sufficiently trivial apps are cheap enough to write that there's no real economic penalty for writing them for WP over iOS or Android, which is why the Windows App Store is stuffed with tiny, trivial apps that barely rise to the level of a fart app.