A one-liner in the Windows Store policy is the reason why we'll never have the "original" Chrome, Firefox, Opera, or other browsers available through the official Windows Store.
Included in the "Security" section of the Windows Store policy, this line is specifically addressed at browsers, and reads the following:
The policy is similar to what Apple has done with iOS, the company forcing apps to use its web rendering engine to process web content. Google took a harder stance with ChromeOS and forbade other browsers altogether.
In both cases, the companies cited security concerns, as their engineers have worked to secure their operating system around those web rendering engines.
In the case of the Windows Store, this revelation came to light after the launch of Windows 10 S, a tethered version of Windows 10 that will only allow the installation of Windows Store apps.
As users kept asking when will Google and Firefox port their browsers for the Windows Store — so they could use them in Windows 10 S — the question was answered last week when a developer tried to convert his Chromium-based browser to an .appx version, compatible with Windows Store distribution.
The Windows Store crew specifically told the developer that they cannot approve his browser because of the aforementioned policy that mandates that all apps that access the Internet use the approved HTML and JS rendering engines. The developer shared his experience and official communications with a ZDNet journalist.
While some were hoping to see Chrome or Firefox available on the Windows Store as UWP apps, this may never be possible, as this would mean that Google, Firefox, and other vendors would need to rewrite their browsers from scratch to use Microsoft's EdgeHTML.
This will never happen unless Windows 10 S becomes a huge success and browser vendors see a benefit to port their browsers.
In this case, we still won't see UWP versions of the original Chrome and Firefox engines, but only so-called bastard browsers, like we have on iOS.
For example, you can't call Firefox for iOS a true Firefox browser, as it's just an older version of the WebKit engine with a Firefox lookalike UI on top, and lacking many of Firefox's original features.
The conclusion is that Microsoft has effectively banned any self-standing third-party browser from the Windows Store. Additionally, Windows 10 S users better get used to using Edge or any of the other bastard browsers that use Edge's re-packaged core. This is why, Windows 10 S won't let you change your default browser away from Edge, or your search provider away from Bing.
The "original" Chrome and Firefox browsers — built around their native engines — will remain accessible to Windows 10 users, via standalone installers only.