I think I understand why there are fewer in the evening peak.
When the full Thameslink Programme was designed, the idea was that the Kentish Town stoppers would be paired with the Sutton/Wimbledon services like they have been for many years. Unfortunately the original design didn’t take the East Midlands services into consideration and had to be tweaked so that in the evening peak the stopping services were paired with the Rainham and Orpington services. That still gave 4 trains per hour.
But the Orpington services aren’t running because of covid, so you only get 2tph in the evening peak at Kentish Town, Cricklewood and Hendon.
Short version : its a hangover from the May 2018 timetable disaster.