Because it has very few advantages over it's competitors and tons of drawbacks.

Windows Phone has a talented team of designers, an innovative and easy to use UI, and backing from a major corporation with practically unlimited funds. It also has a tiny sliver of market share. Jolla has great designers, compatibility with Android apps, and killer hardware. Their marketshare is even smaller. Realistically, right now Android and iOS have all the mindshare and there is not a chance in hell that a me-too mobile OS is going to get any meaningful traction.