Title should be "Why do Sunday services start so late FROM A LINE NORTH OF LONDON"
Ummm...
Uckfield - London Bridge
Monday-Friday: 2tph morning peak service, then 1tph, starts at 0516.
Saturday: 1tph, starts at 0634.
(These two are reasonable, though there’s always at least one passenger who wants the train crew to get up earlier and “why can’t you be bothered to run one for me at 4am?”

)
And then...
Sunday: 1tph only as far as Oxted (most of the day), starts at 1034.
No wonder those trains often cart fresh air around on Sundays...
On a parallel route:
East Grinstead - London Bridge / Victoria (Victoria only on weekends)
Monday-Friday: 4-5tph morning peak service, starts at 0545 (could conceivably fill trains as early as 0530 though)
Saturday: 2tph through the day, starts at 0637
Sunday: 1tph starts at 0820, followed by 2tph from 0912