Some of it will be because Avanti is presently dire and best avoided, but it'll also be because business travel has reduced, probably permanently, and leisure travellers are more interested in budget options, not least budget options like Chiltern that aren't even much slower.
It almost pushes the point that what the UK's network needs is just loads of cheap and reasonably comfortable capacity to allow lots of people to be carried at reasonable fares.
Post Covid there are new norms gradually becoming established, currently distorted by training and industrial relations issues compounded by rolling stock and infrastruture challenges.
My local line is between Sheffield and Manchester. Thanks to a combination of the factors above the fast services, of TPE in particular, have probably lost over 50% of their former commuter traffic. Typically they take 50-55 minutes. The Northern stopping service takes nearer 80 minutes and is usually significantly cheaper. Ridership is possibly up on pre-Covid as many take that option, even some commuters - or maybe it's not surprising?
A grandson has just reported his weekend rail experience. Newcastle to Canterbury on Friday in a couple of minutes over 4 hours. He was really impressed. Almost all day to get back on Sunday, including over 2 hours stuck at Doncaster! The trials and tribulations within the railway industry are not what he wants to have to contend with when trying to have a nice weekend with friends. (Grandad had suggested deferring a week!)
When railways work to plan they can still be brilliant. If and when they do demand for leisure capacity is going to rapidly exceed supply.