most of the mockumentarys are much funnier than the old sitcoms. yeah shows with live audience were funny like friends, frasier seinfeld but the one's without are much better , the office (my fav) , modern family , parks & recreation , arrested development (which you should really watch).
I don't think friends HAD a live audience, I think it was yet another "canned laughter" show. As to why they have became so popular, well that is what enough people want to watch so that it what is on. I've never watched either of those shows, but seems to me they are just another sitcom. A mockumentary is a fake documentary, done in the style to imply it is a REAL one. I doubt that these shows really fir that.