I think Campbell would have had sufficient nous to know that a resignation was sensible. He could then have continued to pull the strings behind the scene
Yes, Campbell is equally awful but much more wily - Cummings is far more transparent in the shit he pulls and his route-one approach has worked well so far. Cummings presumably couldn't stand to lose all his privileges and status by resigning.
Campbell has been very successful in using the 'poor me' stuff to rebrand himself - 'oh I was an alcoholic and a depressive and that's why I behaved so badly and look here's the Hutton whitewash report that fully exonerates me sob sob'. Cummings isn't up to that.