Mostly because that's what I use and it works well for me. There are too many poorly designed or cheaply made cartridge bearing hubs (even some really expensive ones from the big names) that I avoid them completely. Shimano loose ball cassette freehubs hubs are pretty cheap, bombproof, and easily rebuilt. Phil Wood makes great cartridge bearings but they're seriously expensive.

Contradicting my previous advice, a cartridge bearing headset is fine as long as the bearings are easily replaced. And as long as the bearings are a standard size that you can obtain. The cartridge bearings in a headset should last a several years, so throwing out the bearings and replacing them for a few bucks is not a big deal.