Hardware manufacturers are loath to choose a single-sourced hardware component because of the risks implied if that source discontinues production. For the same reason, manufacturers shouldn't be tied to a single-sourced, proprietary OS simply because their application source code isn't portable to other OSs.
By building applications to the POSIX standards, developers can use OSs from multiple vendors. Application source code can be readily ported from platform to platform and from OS to OS, provided that developers avoid using OS-specific extensions.