Universal Windows apps are meant to run on any Microsoft platform with a Windows 10 core, and so that leaves out Windows RT.
From a company spokesperson:
"Surface (RT, Surface 2 and third-party ARM-based Windows PCs and tablets) only run apps built for RT and not Universal Windows Apps. There are lots of apps that work on RT, so they'll be able to continue to enjoy apps, games, music and movie content.
I've asked Microsoft officials several times whether the company still plans to introduce any Microsoft-made/branded ARM-based tablets or PCs and have gotten no concrete answer beyond "Windows ARM processors have a future." Given the Windows RT dead-end those of us with Windows 8.X RT devices are facing, I'm not thinking that future looks very bright ....