It's not rugby - it's their background! England rugby players are traditionally from the public schools. It's just the Bullingdon Club but with a collar size of 18".
I always thought the "boot money" was a way of levelling the playing field: the English clubs tended to be well supported by big business and the players were well looked after, the Welsh clubs tended to be community-oriented and many of their players were from a very working-class background. That last sentence was, of course, a very sweeping statement, of course, but I think there's quite a bit of truth in it.
Curiously, the great Welsh team of the 1970s was also rather class-stratified: the miners and steel workers tended towards the scrum whereas the doctors, teachers and other graduates tended towards the backs! Obvious exceptions: Phil Bennett was a steel worker, Terry Cobner a teacher (again, from memory).
After a bit of googling, I've come up with the following. Of the Welsh players who went on the British Lions tour in 1971, seven went to university, one had a manual job [Chico Hopkins - scrum half - was a "fitter] and four had unlisted occupations. Of these four, three played for London Welsh - no mines or steel works in London.
Of other Welsh players of the era, Bobby Windsor and Charlie Faulkener were steelworkers and Peter Morgan was a coal merchant but Graham Price went to UWIST and was a civil engineer, Trefor Evans was an estate agent whilst Allan Martin, Clive Shell JJ Williams, Terry Cobner, Stuart Lane, Elgin Rees, Dai Richards, Gareth Davies, Jeff Squire, Gareth Williams, Clive Rees and Jeff Young were all teachers.
Obviously some occupations are unlisted but the only miner I can find is Glyn Shaw [prop for Neath]
Of a comparable England team:- Bob Hiller, Mike Slemen, Alan Old, Fran Cotton, Roger Uttley, Peter Squires, Peter Dixon, John Spencer - teachers, John Pullin, Stack Stevens - farmers, Tony Neary - solicitor, Nigel Horton - policeman, Andy Ripley, David Duckham - bankers, Peter Larter was in the RAF
Sorry, had some time on my hands