The main incentive twirled around UWP mentality, “Write one app that works on Windows, WP and Xbox automagically”.
I think it was a fucking-a-star idea that could gather fresh developers to a big potential userbase. And surprisingly, it worked for a time as well.
But MS again cold-feeted the platform themselves in a short span and scared everyone.
I actually witnessed many brilliant developers wrote their very first C# code with UWP, only to spin out to other platforms later as WinPhone’s apparent neglect. PocketCasts and Flipboard are two that went very successful on other platforms.