Well, other than Game-Freak wanted it that way, there I'd a potential in universe explanation.
The metal coat appears to be some sort if unnatural material, as it changes Pokemon into radically rare and artificial looking forms. As such, it is likely man-made, and while Steelix occur naturally deep beneath the earth, Scizor may itself be a man-made evolution of a natural Pokemon, kind of like Electrivire and Magmortar. As such technology is likely very young and underdeveloped during the time of the Johto games, it is unable to increase Scizor's overall power, just rearrange it and change its typing.
Conventionally speaking however, Scizor may have been intended to be a form-change, but the technology for the game just wasn't that advanced in Gen 2, or it could be that if Scizors stats increased in evolutions the creators feared it would be too powerful.
Anyway just my two cents. Hope this helps.