Why do CVH heads always gunk up so much?

Is it because people use crap oil and don't change it often enough?

Or is it a breather problem?

Just re-done a 1400 cvh head after the cambelt snapped and bent a couple of valves. Only done 89k but there was so much gunk under the rocker cover. All the other cvhs i've ever worked on have been the same.

In comparison, a zetec head of the same mileage will be clean as a whistle?

Any one ever got to the bottom of this?