I always thought it was because the earliest stuff was actually filmed infront of a live audience (Like a theatre) who did laugh, so when switching to non-live-audience stuff, the viewing public would be ‘put off’ by no laughter, so they injected it with canned laughter… then as time went on they realised this was rubbish and stopped it.
But maybe I’m just missing the joke in the previous two comments, I dunno.