With the introduction of Windows 10, Microsoft is making some fundamental changes to how its software functions. Beyond bringing back the Start menu, Microsoft will also be making some significant changes to the way the Windows Store works.
Currently, you can only buy 'modern' apps in the Windows Store but that will all soon change thanks to a blog post that was quickly pulled by Microsoft. Because of that post, we know that non-modern apps will soon be sold in the store.
Per the blog post:
The Windows Store will also support more than just modern apps. It will add desktop apps, as well as other types of digital content. We will provide many different ways to pay for apps. And we'll provide an organization store within the public Windows Store, where an org can place their own curated list of public apps as well as specific line-of-business apps that their employees need.