I've been looking at a few old BARLA books and the leagues/divisions before the NCL was formed and afterwards, there is a notable decline in pub teams around the area where NCL clubs are located.
The decline of pub teams is much more to do with the decline of pubs - people just don't spend anywhere near as much time in them any more. Some readers will know the old east Leeds circuit of pubs, where there were at least a dozen and anything up to about twenty within a square mile or so (see the old clubs thread for some of these, but off the top of my head, the Waterloo, Black Dog, Spring Close, Hampton, Cross Green, Fisherman's Hut, Slip Inn, Prospect/Richmond, Yew Tree). Not to mention the social clubs round there such as East Leeds and others I can't remember.
A lot of these pubs had RL teams, nowadays most of them don't exist any more. The business wasn't there to support them, never mind enough people being regular enough to fill a sports team. The city centre has expanded, land got redeveloped, the people are different, and people do different things, clubs amalgamated or disappeared. It's a quarter of a century since East Leeds and Yew Tree merged, change has been going longer than that.