Something that I've never been able to figure out.

Why is Rugby League generally only popular along the M62 corridor?

I know it is played all over the country / world, but the "better" teams are along this corridor.

The main international teams are generally England, Wales, Australia and New Zealand.

Or am I missing something?

Surely it's just because of the history surrounding how rugby league came into being and the foul and most evil Rah-Rah empire sabotaging us ever since.

Lots of sports around the world have traditonal geographic areas in which they are popular.
Australian rules has been the main sport in a few states in Australia for 150 years and has hardly spread at all.
The former soviet state of Georgia has a traditonal game that has a lot in common wth Rugby that is their national obssession, and is unheard of outside Georgia.
Gaelic football is only played in Ireland.
American football was only played in America till recently (and a variation in Canada).
Even Rugby Union is only a 'big' sport in maybe 5 countries, and only the national sport in one (NZ).

What has changed the equation in the last 30 years or so is international travel and TV. Rugby League had competitions in 5 countries in the late 1990s, and is now approaching 30 countries, and it is also spreading across the countries it is played in.

The best thing League has going for it is that it looks good on TV - you couldn't design a better game for TV in fact - follow the ball with 5 cameras and you don't miss a thing.

Where was it popular prior to the development of the UK motorway network?

The A62 corridor you dolt.