Because it was no longer worth throwing money after. Instead they're working to make sure Microsoft technologies - their accounts, their email, their Office products etc. - work well on iOS and Android devices. They're also working to enable cross-platform mobile development using Microsoft technologies (i.e. they bought Xamarin), which helps with getting app developers using (and paying for) Azure backend services because of course Microsoft provide solid .NET SDKs for Azure services.

I suspect they'll make another effort to break into the mobile device space in the future, but it probably won't be for a while. Where's the marketplace opportunity at the moment? At least until they can distance themselves from the current perception of their mobile experience.