I think that the ever-tightening rules on what the advertising censors will allow are responsible. Many of the funny adverts from the past would not now be allowed. They can't make alcohol look pleasurable these days, for instance.
And it only takes a few telephone objections for an ad to be pulled - look at that hilarious Christmas ad for Mr Kipling cakes, which had to be taken off the air. In that case, the objections were not from people who had found it offensive to send up a school nativity play, but from those who thought that OTHERS would find it offensive.