The Town class was too big and expensive to play the same main roles of the Leander and Arethusa classes(trade protection and foreign station service). La Argentina wasn't(it was getting a bit big for the role, though, which is why I feel that the equivalent of La Argentina must not be too big).
Except
Arethusa and
Leander didn't occupy the same role in the first place. The
Arethusa class was a reduced size "trade route protection" cruiser, but the much larger
Leander class was a full-fledged fleet cruiser.
La Argentina is an enlarged
Arethusa class but remains considerably smaller than the
Leander class.
The
Town class directly replaced the
Leanders as fleet light cruisers
, and also replaced the
Arethusas because the "trade route protection" role was judged to have been a mistake; the
Arethusas were used as fleet light cruisers for pretty much their entire careers, albeit painfully cramped and undergunned ones.