Why naval treaties needed a destroyer-leader category

Why not give every destroyer the same treaty tonnage limit?

Because flotilla leaders needed extra command, communications, accommodation, and often armament, so navies already treated them as a distinct type.

The Washington system initially placed no numerical limit on ships below 10,000 tons, but later treaty categories had to divide that broad field. A flotilla leader was larger than an ordinary destroyer because it carried the personnel and communications needed to command other ships. It was therefore neither a normal destroyer nor a cruiser, even when its weapons looked similar to those of its flotilla. The category reflects an operational command requirement rather than arbitrary treaty drafting.