I don't know the population of the entire London region as opposed to Greater London, but I can't think it is more than the 15 million conceded by Grayling as the population of the 'North". Crossrail alone is estimated to cost almost 15 billion. What is the government prepared to spend on 'Crossrail for the North'?
The problem with comparing population numbers is that, although to get to 15 million you need to include London and the South East, is that the number of rail passengers is vastly different.
Something like 2/3 of so rail travel is classed as London and the South East. Note that Intercity travel that originated in London is not counted in those numbers.
There are 4 coach trains in the South East that are on services where there are 4tph in the off peak periods have no spare seats, both into and out of London.
As such if you spend £15 billion on rail improvements in the North you would need passenger numbers to go through the roof to justify that level of spending. As otherwise there's going to be significant numbers of under utilised infrastructure. Which would result in the subsidiary for rail services increasing. Conversely TfL needs to break even (mostly by funding it's own improvements from fares) and so the business case for Crossrail 2 had to more than cover it's running costs.
Also it is worth noting that Crossrail 2 had the potential to benefit London, the South East, the East and parts of the South West. The latter being the potential to have more services from London heading to/through Salisbury.
One last question, which region benefited from the revolt of Reading? As it is clearly spending within the South East, yet it will allow more services to Wales and the South West.
That's not to say that there shouldn't be investment outside of London, for there should be, rather people shouldn't be looking at what one region has and say "we need X so that we receive the same" bit rather should be saying " what needs to be done to provide us with a good rail service?" As the two are very different answers.