MS provides support for its products even when they have signally failed in the marketplace, like Win 10 Mobile. Contrarily folks bitch when they DON'T provide patches for old out-of-support software -- there's a current furore over a security hole in Win2003R2 Server which MS isn't patching since they stopped support for that particular version over two years ago.
Regular Win 10 runs perfectly well on phone-sized systems such as tablets without the hardware limitations of phones two generations back (ARM CPUs with one or two cores, limited memory, limited storage etc.) so Win 10 Mobile isn't really needed but there are still customers out there who do use it. It will reach EOL and support will eventually be terminated but there's no rush.