It seems that there is no push to ever change such things. If NR can cope with the route being open at that time on a Sunday morning in the summer - when the fens often have the most issues (see the current issues on the Kings Lynn line) then I don't see why they can't all year around, when this would be highly benefical to passengers.
Currently you can get to Leeds from Ely at 1122, or Newcastle at 1222, on a Sunday morning when this service runs. Without it those times are 1323 and 1433, which is a massive difference if you're hoping to do some shopping or tourist attractions or similar before everything closes down until Monday. In the December timetable the connections are better at PBO so Newcastle can be reached by 1400 (no change for Leeds), but that's still quite late.
It's possible when we see the GN timetable that the promised earlier start for KGX services will allow for earlier opportunities via Stevenage, I suppose. Let's see once everything is published.