It has been noted that the city of Liverpool produces a lot of comedians and that there is an identifiable, unique, sense of humour that people in the city possess.
Liverpool received a lot of Irish immigration in the late 1800s and this strongly defines the identity of the city and the people there, the "Scousers". I believe that Liverpool's sense of humour is due to a unique combination of Irish humour, which has a madcap, surrealist bent to it, and the sense of humour of the North of England, which involves engaging in witty and affable ridicule and put-down. Nowhere else in Britain, Ireland or anywhere else have these two elements of humour been combined and I believe this is what gives Liverpool its unique sense of humour. It is unlikely to be the case the people in the city are funnier than anywhere else, but the unique flavour of comedy emanating from there probably makes comedians stand out more and achieve greater attention and success as a result.