It was that now rare thing, a good sitcom. Ran from 1981 to a couple of years ago.
It works because the characters are not perfect, and are essentially trapped by their circumstances. It's hard to make funny situations out of perfect, contented people.
It's really like all good sitcoms, a polar opposite of 'Friends'.
But that's maybe the British liking for often rather bad-taste, politically incorrect humour, that hasn't got to have a patronising 'moral' written into every storyline.
That's not USA bashing, sadly most British TV sitcoms are more like Friends these days.