Yeah it seems like they want it as "that stone that split an evolution line into very gendered directions" so they probably wouldn't want to use it for "just" a gendered evolution (ie: salandit to salazzle) and would be weird for what's basically just a standard gender form change (meowstic line) and it'd be weird (even despite their recent slight retconning of methods) to use it for a species that already existed while the dawn stone existed (ie: what if we got a femme Machoke split). And designing a new species that did fit that criteria would probably be weird when they can just...do a gender split evo or a gendered form or have an entirely unique evolution method.It's pretty odd they never came up with another use for the Dawn Stone. Immediately in Gen 5 they found new uses for the Shiny and Dusk stones and later gens would continue to do so as well; and even Gen 7's Ice Stone would get new uses. To say nothing of course of Gen 1's stones and the Sun Stone. But out of all the evolutionary stones, the Dawn Stone has uniquely only ever had a use for Pokémon introduced in its generation; and only two at that.
It's not even like they strayed away from gender-specific evolutions in later gens either, with Pokémon like Meowstic and Salazzle. I wonder if they just want to keep it to its very specific use (new cross-gen split evolution specific to one gender) and have never come up with anything like that since Gallade and Froslass that they really want to do.
They basically made themselves a design conceit maze to navigate and just don't want to bother coming out of it.