This illustrates a problem with a fragmented railway, where the infrastructure is built separate to the operation of trains.
At the moment, the EMT franchise is running a lot of five/seven coach 222s from the East Midlands to London (plus a lower number of HSTs). At the risk of sounding patronising, a five/seven coach 222 clearly doesn't have as many seats as a five/coach 125/225, so there's not a lot of spare seats for EMT to try to fill with "Parkway" passengers.
In a few years time, the MML will hopefully have electric stock comparable in length to 225s/390s on the parallel EMCL/WCML, which means that the successor TOC will have an incentive to stop more services at East Midlands Parkway (both for people heading to London and for people using it as a Park And Ride for Nottingham/Derby/ Leicester)...
...but at the moment there's not a lot of spare seats on the MML, so little incentive for EMT to stop more trains at East Midlands Parkway/ little chance for them to sell more advance tickets to tempt car drivers from the Motorway.
In a few years time, we could be looking at longer trains providing a "turn up and go" frequency from EMP to Nottingham/ Loughborough/ Leicester/ London (with more cheap tickets for longer distance trains).