Why treaty-era sloops stayed below 2,000 tons

Why did interwar navies keep sloops below the 2,000-ton treaty limit when other ship types were built to their limits?

Because the limit was only a ceiling, while sloops were meant to be cheap and did not need the size or speed that would justify using the full allowance.

A treaty ceiling is not a design target. Sloops were intended to be inexpensive, workmanlike ships, so using the full 2,000-ton allowance would undermine the reason for building them. Staying well below the ceiling could also avoid drawing attention to treaty wording that navies found useful. Their modest performance requirements, including no need for a 20-knot limiting speed in the example discussed, reinforced the case for a smaller ship.