I think that's mostly due to marketing, and history. I mean, 1966:
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How and Entwistle popularised the stainless steel roundwound thing, a lot – apparently, or according to Wikipedia, The Who even included a fake Rotosound jingle on one of their albums. With Rotosound being a British company, their strings being readily available on the Isles, and the whole NWOBHM thing, it sort of crept in. And stuck.
Even nowadays, string makers and other companies market strings thusly, say, d'Addario XL Prosteels: “Brighter, crunchier tone”. Or Carvin claims “Stainless steel or roundwound strings, for instance, have a lot more brightness and punch than the warmer, rounder sound provided by flatwound strings.” Adjectives such as “crunchy” or “punchy” etc. sound more interesting to many metalheads than “warmer” or “rounder”.