I imagine it comes from the fact that the South of England TOCs do not, in the main, overlap, whereas TPE and Northern run many services between the same destinations.Why is it that the South of England is regarded (rightly in my view) as needing multiple TOCs, partly in order that they don't become too large to manage well, yet it is even considered that one TOC would be suitable for the North of England?
I guess people think there could be efficiencies if the operations could be consolidated, but Northern hasn't itself achieved that level of harmonisation yet so holding out hope of bringing all of Northern and TPE together seems a sizable challenge.