Thameslink after 1am from Blackfriars

is there any reason why Thameslink services going southbound to Brighton / Three Bridges from Blackfriars do not go via London Bridge (without stopping). Obviously, going via the south London route takes a lot longer and I just wondered what the reason was for not routing them through London Bridge during the early hours?

It allows the London Bridge route to be closed for engineering work when necessary

As above, there’s a possession lines 4+5 most nights of the week. They do occasionally go via London Bridge overnight. The possessions are normally booked so there is one diversionary route open. IIRC there is one night when the snow hills are blocked to Blackfriars Junction so there is no diversionary routes.

I’ve seen it a fair few times when the Thameslink go via Met Junction and platforms 7 and 8.