I don't know the exact situations behind HorizonXP's company and their refusal, but having worked in devrel on two minority platforms, I can share with you some common reasons:
-[small companies] even if you bought them a developer, it would impede their ability to be agile and pivot and iterate as they try to find something that works. It's hard enough to get them to support Android in addition to iOS, but getting them to support WP or BB10 is even harder
-[big companies] don't need the money. Or would ask for outrageous fees (some companies have asked for north of $20 million, even for faddish apps)
-[big companies] have an existing solution to do cross-platform development for iOS and Android and it's not worth it to redo it to add WP/BB support
-[small companies] complete lack of familiarity with the Microsoft stack.
-pressure from Apple or Google (usually Apple). I've had cases where a company gets favorable placement in the app store or in a commercial, or placement on the demo devices in a retail store and planned Windows ports are put "on ice" because they don't want to jeopardize their relationship with Apple (which of course has reserved the right to be as capricious and arbitrary with its app policies as they like)
-personal dislike. Believe it or not, some companies are run by people who just have a personal dislike of BlackBerry or Microsoft that is so strong that they'd rather spite them than add to their customer base. It's incredibly immature, but nothing we can do about it.