Most CP/M software tried to do a lot less than MS-DOS/PC-DOS software did, which made CP/M a lot simpler and easier to manage in practice than DOS was. The vast majority of CP/M applications were purely text mode, no graphics (especially not on the screen, printer/plotter graphics was somewhat more common), no sound (except the terminal bell), only supported IO devices were floppy/hard disks, and serial IO to talk to printers/plotters/modems; they managed to get by with very limited memory (many CP/M systems only had 64KB of memory, with some of that reserved for the OS or hardware; higher-end systems with bank switching could have 128KB, occasionally even up to 512KB). If your DOS applications stuck to the same limited feature set, DOS could be about as simple.

But, people came to expect a lot more from DOS than that, and DOS' architecture had never been designed with those greater expectations in mind, so the end result was a complicated and difficult to manage mess. CP/M's architecture was fundamentally no better (indeed, DOS 1.0's architecture was just a slightly evolved copy of CP/M's): if people had tried to make CP/M do the things which late 1980s / early 1990s DOS systems were expected to do, CP/M would have turned into just as big a mess as DOS did.