I've noticed in a couple of bus networks that Saturday services to (relatively) small towns have been reduced but Saturday services to bigger cities (or almost-cities) have been maintained or increased in the last year or so.
It may partly reflect the demographics of who's going shopping where and when - do the smaller towns attract a higher proportion of older people who - if retired - will probably try and do their shopping in the week rather than on Saturdays, and if they do go out on Saturdays tend (as a generalisation) to want to be home again by lunch time? And maybe younger people are more likely to start that bit later and stay out in to the afternoon - but want to go to a bigger town?
Wouldn't be more surprised if it's because people in the smaller towns all have one or two family members who drive to work as they work somewhere without a sensible way to commute there by public transport whereas the bigger towns have more families without a car or only the one.
As a result, on a weekend they just jump in their car and go shopping at the nearest retail park or whatever whereas the bigger towns, people stick to public transport and without the students and pensioners using the buses there's little demand.
That's before going into discussions that a lot of smaller towns are terrible to shop at so unless you had no other choice, you'd avoid it. The North East is plagued with bus routes going to towns like Bishop Auckland where the place is a ghost town and there's no shops anymore, while everyone with a car is outside of town at the new retail park. Teesside Park is another notorious one with an absolutely dreadful public transport offering.