Game Freak logic. Though I have turned into someone who has theories that don’t make sense, so here you go.
Theory #1: The shiny stone may make Roselia evolve into Roserade as a nod to Roserade’s name. The “rade” comes from “masquerade”, or “a false show”. If you think about that dictionary entry, the “false show” could be a reference to a movie, or a movie star. And you know what stars do? They shine. Like a Shiny Stone. Also, actors PRETEND to be their characters.
Theory #2: This theory is also that the Shiny Stone links to Roserade’s name. Again, the “rade” part. If you think about it, a masquerade is when you pretend to be something you aren’t. Maybe, just maybe, the Shiny Stone is so shiny and bright that it somehow conceals Roselia, making it evolve into Roserade.
Theory #3: The Shiny Stone has light in it, kind of like the sun. So Roselia evolves from the light emitting from a Shiny Stone, because maybe it is brighter than the light coming from a Sun Stone (Whimsicott-W’s theory, by the way, I’m not going to take credit for it).
So, in conclusion, the answer is Game Freak logic, but if you want, you can just make up your own theory for it or use one of these.
Note that these are not official and were created by trying to make the most reasonable guess for it.
Hope this helps! :)