Comedy is entirely subjective, but a massive number of people love FT - I am one of them I have to say.
It's pointless trying to explain what one person finds funny to someone who doesn't share their humour, but the two series have grown to iconic status over the thirty years, hence the renewed interest in them.
For the record, ignore the slapstick elements and look at the subtle cluster-bombs of plot that are launched as each episode develops, and see them explode by the end.