The question is not why some use angled drivers. The question is why so many do not.
Your ears are angled. Some are bigger than others. Some are angled more than others. But none sit flat against your head. Yet so many headphones are built as if your ears were an afterthought.
The smaller headphones can sit upon the ears. It helps if they swivel. With the larger headphones, so many of the circumaural designs ignore the need to orient the driver to match the ears.
Why?
Because the purpose of a headphone manufacturer is to sell headphones. Making a great product is part of that, but far too often, the object is to sell a cool looking set of cans. Angled cups don't look right. The vice-grip look is iconic. People are used to seeing headphones as sitting flush with the skull. They're not used to seeing headphone backs as something staring out from the face. Some time ago, I made some modifications to the cushion design on a pair of Grados. When I took a picture of the cushions with the drivers angled, I got a lot of snickers for doing so. But that's exactly what Sennheiser has done with its HD800 - and good for Sennheiser!
The HD800 may be the headphone that starts a revolution in headfi, with an army of me-too knockoffs imitating the angled drivers, among other things. When I saw that the photos of the HD800 included shots of the rear of the driver, which shows a widening in the rear (to accommodate for the angling of the pinnae), I thought it was marvelous. Finally, somebody has made a headphone that has made it cool to do what should have been done long ago.