CP/M is popular because its requirements are dirt simple, and it has a lot of software that still runs. It's popular because, unlike IBM PCs, the hardware was diverse, and the software has to cope with that. Now, while MS-DOS is very CP/M-esque in its design, the truth is that most MS-DOS software is actually PC software. It assumes the PC hardware. Most major MS-DOS applications skipped the MS-DOS driver part and just went straight into the hardware. Today that means if you want an "MS-DOS" machine, you really need a PC with all of its hardware, its graphics, its ports in the right place, etc.

In contrast to CP/M. It requires a block device of some kind (i.e. a disk drive), and a console of some kind (typically, but not necessarily, a serial terminal).

While some applications did route around the BIOS and BDOS in CP/M, most did not. CP/M was quite portable across a crazy range of hardware. Porting a CP/M BIOS is a nice engineering and assembly language challenge for anyone willing to take it on, depending on their hardware environment.

Today, anyone with a Z80 chip, a RAM chip, and some paperclips to wire it all together can get a device that will run Wordstar. That flexibility makes CP/M very popular with retro enthusiasts today. It's easy to find dongle sized computers for $50 running CP/M on eBay. It's also easy to find full through hole kits of parts and PC boards to make your own. Or, grab a Z80 CPU and Peripheral manual, a copy of KiCad, some parts and breadboards and go to town. Having your creating run Wordstar beats blinking LEDs any day of the week.