In message , Basil Jet
writes

Why are there so many "commons" south of the river and so many "parks" north
of the river?

Probably because better soils south of the river meant that South London
supplied the bulk of produce to the city until the 19th century. While a
lot of this was market gardening, most parishes also had common land for
grazing of animals that went to London markets.

Poorer soils north of the river meant that common land was less valuable
and, before the various late-19th century acts to preserve commons, was
more easily enclosed and built over. For example, the original extent of
Canons Park, laid out by the Duke of Chandos, included the enclosure of
Stanmore Common. Much of Old Oak Common was purchased for railway use,
and so on.

However, I doubt that there are significantly more parks north of the
river. Names can be deceptive: Richmond Park was originally common land
before it was enclosed, while Hampstead Heath, Hackney Marshes and
Epping Forest are all, in effect, commons.
--
Paul Terry