That is a strange comparison. As said before, it is 4 tracks but, also Sydenham & FH don't have platforms on the fasts. So GTR doesn't have a choice but to run fast for all but 4tph. After all the Thameslink works, the Sydenham corridor is probably the only place along the Thameslink network that has a negative or no improvement out of the Thameslink Programme. At a time when everything is getting later, even our very very popular last service from London Bridge is planned to be withdrawn for an earlier one, so I'd think the North Kent should be happy considering its getting Thameslink and a Crossrail interchange.
Herne Hill is two stations from London Victoria, its hardly a long journey. Brixton and West Dulwich have become busier, quite a lot of people get off at Brixton from the south. Like all along the route Sydenham, Penge and Beckenham are South London suburbs, in zone 3-4. Again, not very out and only 20-ish minutes into Victoria, they aren't all the way out like Shepperton. They aren't nearly as busy as Putney, Richmond or Twickenham, they just don't warrant semi-fast trains.