It is really quite obvious. As others have explained MK has more jobs that attract commuters. I know many people who work in MK but live in Northampton. They either drive or use the train to get to work. People aren't commuting into Northampton in the same way.
Put simply MK has more company head office/HQ type offices ( not just those mentioned above but off the top of my head Argos, Mercedes, VW, Deloites, Dentons) which pull in a lot of professional people, from a wide area, by both train and car. Some will come by bus but not that many. Northampton doesn't (outside Nationwide and Barclaycard) have the same level of these jobs or companies located there.
I would also venture that more people live in Northampton because it is ( or perhaps was) much cheaper than other areas for housing. I have been here for 15 years now and Northampton was the affordable London commuter boundary for a long time. It is now further north.
Even with that the demand for commuting (pre covid) was satisfied by the service offered. It wasn't a drag on usage!
Bedford direction is another one. Hopefully E > W Pick both up
Knowing the area is not enough to base an opinion on
lets not get carried away!
No they are not, at least not beyond long term idea generation. A what could we do IF session I suspect.