I sort of get that’s because of signalling limitations but on other routes it’s quite poor it’s gone on so long considering the popularity of Sunday rail travel.
I have an Eastern Region London Suburban services booklet dated
May 1973 and the first Up service from
Huntingdon on a Sunday morning was at
09:05 and that only went as far at
Hitchin, to connect with a train from Cambridge to Kings Cross. The next UP service was not until
15:38 but at least that was a through service to Kings Cross, there were only another 3 services to Kings Cross, so only 5 services in total and one of those wasn't direct. And that was on the ECML a premier mainline, so what hope had lesser used routes.
The first Up service on Sunday in the current timetable (pre-22 February and from 5 Aprii) is
05:58 so over 3 hours earlier than in 1973 and now there is an hourly service with 3 extra faster services in the morning.
Don't forget that Sunday trading is recent in railway terms (only permitted from 26th August 1994) and therefore the timetable specifications for the TOCs at privatisation would have been based on pre-Sunday trading usage. Nobody was willing to pay for extra services on Sunday mornings, as that would have meant additional shifts being required.
It is only with the advent of large signalling centres that allowed earlier Sunday services to operate.