Well obviously its not scientific but just a trend in my opinion. Perhaps going back 40 years doesn't do it justice. Football didn't start in 1975. How long is it since, Preston, Burnley, Huddersfield, Bolton, Blades, Wendy, S'land, Newcastle Leeds were winning things ? Ok they all have their moments but Huddersfield won 3 titles in succession, Burnley during the 1960s were always up there, Wolves in the 1950's.
Over the last 50 or so years, Swindon, Oxford, QPR, Norwich, Luton, W'don have all won cups what about Hudders, Preston, Burnley, nothing. There are examples that buck the trend eg. M'boro winning the league cup.
Eventually as wealth becomes ever more concentrated in the south of England I think the Merseyside clubs will decline as a lot of the top players seem to want to play in London. Chelsea, Arsenal, Wham (new stadium) Spuds, Bournemouth, Watford , Reading (recent years), Southampton.
I think we have to accept, unfortunately that clubs like ours, Huddrs, Wendy, Leeds, S'land, Newcastle are on the wrong side of history and geography,
What I think you are actually seeing is a consolidation of power in the big urban centres. As I said earlier it's now virtually impossible from any club outside the big conurbations to win the league (or indeed get into the top 4 in the PL - from memory, apart from Blackburn, the only clubs to have qualified for the CL are Everton, Liverpool, Man Utd, Man City, Spurs, Arsenal, Chelsea, Leeds and Newcastle).
Clubs from outside those centres can win the odd cup , but I see no distinction between the "traditional" football areas and S in that respect, in the period since 1969 (when Swindon won a cup), Sunderland, Wigan, Wednesday, Stoke, Wolves, Coventry, Birmingham, Villa, Middlesbrough and Forest have all also won cups. The southern clubs you mentioned have won a total of 6 cups whereas the clubs I mention have won a total of around 17. Also, of course, Derby, Villa and Forest won the league in that 50 year period. It's a fact that only 1 club from the south oustide London has won the League since 1950 (Ipswich in 1962) and there is no prospect of any other club doing so any time soon.
There is a marginal shift in power to the south, but all it is clubs like Luton andNorwich getting a few years as top tier strugglers and winners of a cup every 10 years or so (but then clubs like Blackpool, Bolton and Burnley have all also managed that in recent years). To go back to the OP, power in football will always go to those with the biggest crowds - and that means the clubs from the big urban centres (apart from Sheffield of course :-()