8 bit CPUs are seriously limited and the OS on them needed to be user friendly and lean. CP/M mostly covered that. The other thing is, many early computer users also had used mainframes, with DEC being exceedingly popular in colleges and universities. Among the operating systems for DEC’s machines was TOPS-10, from which CP/M drew a ton of inspiration. CP/M being similar to TOPS in many ways made it easy for people to pick up. There’s also timing. CP/M had serious first mover advantage.