Why light cruisers did not grow to the treaty maximum

Why would Britain not build much larger light cruisers if heavy cruisers were allowed 15,500 tons and 9.2-inch guns?

Because light cruisers were procured for numbers and worldwide trade-protection tasks, not to consume the maximum allowed tonnage.

Light cruisers had to cover commerce, convoy defense, and economic warfare across a global system, making numbers central to their value. Some designs such as the Town class could grow, but routinely building to a 15,500-ton heavy-cruiser ceiling would sacrifice too much coverage.