Mostly because there were no contemporaries. HDOS (Heathkit DOS) was an alternative for a while (only on Heath/Zenith) but almost all third party packages were for CP/M. It is hard to explain the hardware diversity, even in the 8080/Z80 universe, but relative lack of software diversity (there was no IBM PC standard). But if you wanted to run WordStar, CP/M was the only option.