Interesting to see the differences between the MML and ECML in this regard.
The northern terminus of Thameslink trains on both routes are Bedford and Peterborough, but Peterborough is further out and comparable in distance to Kettering.
So, yeah, I see why the Nottinghams are first stop Kettering, from a distance perspective. I am sure that ECML intercity trains would be much slower if they would be first stop Sandy and all stations to Doncaster.
Just because Sandy happens to be a similar distance from the London terminus as Bedford means nothing. The significant populations (in terms of rail travel) on the MML and the ECML are totally different north of Hitchen/Stevenage and Luton respectively. (And if you live in Huntingdon as implied by your avatar, you should surely know that.)
This was reflectied historically, when, until electrification, Biggleswade/Sandy/St Neots had an almost non-existant service outside the peaks - in complete contrast to Bedford and Wellingboro'. (I suspect home-building may have changed the proportions over the years, however.)
Given electrification and 2TPH to Corby, I think it's quite understandable that planners would want Bedford and Wellingborogh passengers when heading north to change at Kettering for much of the day (and certainly during the peaks), and v v, but to have effectively no direct services for the vast majority of the day is just wrong IMO, and will naturally push people from these intermediate towns (not to mention Flitwick folks etc) into their cars.
People, or 99% of normals, simply don't like changing and all the worries and uncertainty that changing trains entails. If Chief Planner, with free tickets and expert knowledge of the services, platforms etc, is put off by three changes, getting Joe Bloggs in Harlington, or even Aunt Maud in Bedford, to go to Leicester and onwards by train is going to be simply impossible.