I don’t know the material science so I may be a bit off base, but but since kevlar is a synthetic fiber I’m assuming it’s made from petrochemicals and probably a source of microplastic pollution. So there’s probably an environmental aspect to consider. Even if it’s not actually more effective for it’s role, the fact that it’s tougher could be a useful marketing buzzword to get people to switch who wouldn’t care about the environmental impact and maybe even justify an inflated price.

Also while body armor is probably by far the best known use for kevlar it does have other uses like in composite materials in a similar fashion to things like fiberglass. Shaving a couple ounces off a part that might otherwise have been made of kevlar by switching to silk might be attractive to certain fields, maybe we’d see high end racing sailboats with silk sails or something (not that I think that’s a very important use, but for some reason I know some boats have kevlar sails so that came to mind)