Just looking for some times from Northampton, northwards to Leicester or Nottingham and I see that in almost every case you have to travel south to Milton Keynes Coachway and change before heading north. The resultant journey times are three and a half hours or more which makes them not at all attractive.
It's a bit ironic that Northampton had far better express services in the old days of United Counties and the MX1 / MX5 services from Nottingham to London!
I suspect on Northampton to Leicester, NX don't think they can compete with a half hourly express bus taking 80 minutes.
https://tiscon-maps-stagecoachbus.s3.amazonaws.com/Timetables/Midlands/Northampton Timetables/31st Dec 17 & 2nd Jan 18 Changes/X7 NORTH WEB 021118.pdf