Why does this general area appear to be the railway equivalent of the Bermuda Triangle in terms of frequency of disruption?
See also "Lewisham" for Southeastern. I imagine it's a somewhat complicated junction.
Total lack of investment as treated like a little used branch line for Maintenance? Probably not but it does feel that way and Thameslink seem to consider stations they provide 75-90% of service at as Southern stations in management terms so don't care if trains don't call, it someone else's problems (and yes, I know Southern and Thameslink are effectively the same company sharing resources but in operational management they are not)
Also, Go-ahead owns the local buses so are really happy to have Rail passengers fill them up to create more income.
Control is indeed shared at Three Bridges, so that argument doesn't really hold much weight. Nor does the "buses" argument as they quite far apart in reality from the day-to-day.