Why pre-Fletcher destroyers came in many small classes

Why did the United States build many small destroyer classes before the Fletcher class?

They were iterative batches that incorporated new technology, threat assumptions, and lessons discovered during design and construction.

Successive destroyer batches incorporated changing technology, threat assumptions and lessons from earlier ships. A visible change such as enclosing a previously open gun mounting could make an important operational difference. Full-size drawings in the mould loft also exposed problems that smaller plans had missed. Building ships therefore generated knowledge that could feed into the following order. The Royal Navy's A-I classes provide a similar example of useful batches that advanced cautiously, increasing a design parameter after the previous version had proved workable.