Most of the easy street parking is occupied well before 8am. It has been like this since Perry Vale car park stopped being free all day. But I don't think parking is the reason it is so popular.
The catchment for Forest Hill is big. If you consider how close Crofton Park and Honor Oak Park are, or how close the two Catford stations are, or that Sydenham has 3 stations, Penge has 2, and Dulwich has 3, you begin to understand how wide an area Forest Hill serves.
Our station also has more regular services than stations to the east and west, and is well connected to a variety of bus routes.
Forest Hill is also a destination due to the Horniman and that boosts numbers compared to other stations in south east London.