It's not just 'beyond' Leeds. It's most of it. And it's really all down to history. Just as the GC suffered from being the fourth line from the north into London, the Midland suffered for being the third - although the GC was built 'as one route' except for the Metropolitan bit from Aylesbury.
The two prime lines, (what became) the LNW and (especially) the GN, took the plumb routes and plumb traffic centres.
The Midland was built 'going off at an angle' in the first place - the route to London from Derby originally feeding the LNW at Hampton in Arden. Then it got routed via Leicester to Rugby, then via Bedford to Hitchen, and finally the directors bit the bullet and built the London extension to St Pancras in the 1860s. But all those compromises left the Midland with speed restrictions on what is now the main line to london, starting at Derby and including almost everywhere of importance and some of almost no importance (eg Trent, Wigston and even M Harborough).
In steam days, with top speeds of 75 mph or so, these didn't count too much, except perhaps for Glasgow and Edinburgh. But once the GN got Deltics and the LNW got 100 mph electrics, the Midland was exposed with significant timings disadvantage. Plus you've got to add in the fact that little was done to eliminate these horrid PSRs (unlike, say the LNW, where horrid sums were spent in the 70s to elminate eg the 90 mph at Wolverton, all for, at that time anyway, a 10 mph increase).
Much as I love the line, and certainly once it lost being a serious contender for Leeds and Manchester traffic, I don't think it can be compared of the same importance as the GWML, the spine of which serves Bristol/Bath and the longer-distance S Wales and West of England cities plus feeders like Gloucester/Cheltenham, Oxford and Worcester.
