It all goes back to the days of Rugby Union. Back in the late 1800's and early 1900's the Northern teams couldn't afford to travel down south for away games so one sunny afternoon in Hudderfield, hey had a meeting in the George Hotel (outside the train station). They broke away from Rugby Union to form the Northern Union - later named Rugby League. As the games have pregressed the sports have changed to be nothing like each other.
Castleford, Leeds, Wakefield and Featherstone have always played the sport and because there is only one football team in this area, most lads seem to play Rugby League. After this area, it's not far out to Bradford, Dewsbury, Batley and Keighley. It's such a strong base for the sport.
Across the pennines it's the same with St Helens, Warrington, Wigan, Widnes and Leigh, you can even put Salford into the frame there. There is a lot more football going on there though so they don't breed as many players as we do here, as shown in Yorkshire V Lancashire games.
Hopefully that helps, lol.