Probably because they are cheap and easy to fit, look at how hubs have evolved, up to the boom in CNC manufactures from the late 80's -late 90's, most hubs were cup and cone, then came CNC, and everyone seemed to be making hubs, and most/almost all were cartridge bearing, now most of those manufacturers are long gone/names taken over.

From a consumer POV, cartridge bearing are easy to replace at home (yes you really need a few tools like a bearing press, but this is a one time purchase and if you have a FS MTB, you will probably use this for that as well) need no adjustment and are pretty cheap

For revenue stream, given that most bearing are standard industry size (not just bike industry) and you can go to any bearing shop and get what you need, no manufacturer of hubs get any additional revenue stream unless the purchases insists on buying the bike 'specific' part rather then the exact same non-bike specific part.