Sheffield might be 70% the size of Leeds, but LNER Leeds trains also act as the railhead for all of West Yorkshire- pop 2.3m and some parts of North Yorkshire too. London-Leeds trains are far busier than London-Sheffield trains.
The KX-Leeds service services only Leeds (plus a few other smaller towns like Wakefield, with places like Doncaster and Peterborough there are plenty of other services to choose from). The MML also serves the significant settlements of Nottingham, Leicester and Derby. They serve as railheads for most of Derbyshire, Nottinghamshire and Leicestershire. I'm comparing the
entire MML to the KX-Leeds services, not just doing a pointless comparison of Sheffield and Leeds.
The ORR puts the total station usage of Leeds (Entries & Exits + Interchanges) at approximately 33m per year. That includes a great many passengers who won't be using the ECML. A similar total for Nottingham, Derby, Leicester and Sheffield comes out to approximately 28.5m per year (and I suspect a greater proportion of those will be using the MML). Those numbers are, in my opinion, very
comparable (which is what I said, I'm not saying the MML is more important than KX-Leeds, just that the two are comparable, i.e. "in the same ballpark").
The MML is a secondary mainline. It will remain that way until it is electrified or has further line speed improvements.
It's in dire need of both of those. Unfortunately, most government ministers would struggle to locate anywhere north of Watford on a fully labelled map, so have consistently undervalued the majority of England.