Why Leanders were not Town-class substitutes

Did the Leander class influence the Town class?

Not directly; the smaller Leanders protected trade, while Towns and Counties supplied greater presence and fighting power.

The Leanders and Arethusas were compact attempts to stretch treaty tonnage across the Royal Navy's commerce-protection commitments. Their smaller size provided hull numbers, although the service worried that they lacked the strength expected of British cruisers. Town and County classes filled a different role by carrying greater fighting power and acting as conspicuous symbols of British presence overseas. They could deliver that status effect without the expense and escalation of sending a battleship, so the Towns were not simply enlarged substitutes descended from the Leanders.