The fact that they both evolve via exposure to dawn stone is one theory behind it, yes. Dawn stone affects those with abilities that are either waiting to be unlocked completely or that may be boosted through an alternate evolution. Gallade, for example, is the final evolution of a line that’s best known for their fierce loyalty to their trainers and their collective dedication to protecting the ones they love, and as such, gallade is the embodiment of that idea. Froslass, conversely, are wraith-like creatures made of ice and snow, known for their human-like appearances—very much the extension of their childlike preevolution, snorunt. They are the alternate evolution of creatures made of rock and ice, yet they’re hollow by contrast, which is to say froslass is what glalie could be if glalie didn’t seal itself up in armor. So in other words, whereas gallade represents the enhancement of what it means to be a member of the ralts family, froslass represents what snorunt has become underneath the hardened ice-and-rock body of glalie.
That having been said, back to your original question, as I’ve implied above, there are a couple of reasons for why only male kirlia and female snorunt are affected by this stone. The first theory is that only male kirlia and female snorunt may reach the potential outlined above. That is, only male kirlia can experience the need to protect others so strongly that being a gallade is an option for them, and only female snorunt may experience the need to be free so strongly that being a froslass is an option for them.
However, the other theory is a lot simpler: that for these lines, part of the evolutionary requirements are coded by the sex chromosomes. Think of it like binary, where 1 is on and 0 is off. For the snorunt line, this concept is straightforward. If snorunt have two X chromosomes—or two 1s—then that means their ability to evolve by dawn stone is “on,” as they receive two “on” signals. Male snorunt, meanwhile, have one “on” signal and one “off” signal, rendering them incapable of evolving because the requirement for their species is that both switches must be flipped to “on.” Conversely, it’s the reverse for the ralts line. Whereas the question being answered with the snorunt line is “can this pokémon evolve,” the question being answered with the ralts line is “will this pokémon not be affected by this.” Thus, a 1 with the ralts line means part of their resistance to dawn stone is active, but it’s not complete. Two 1s in a row, meanwhile, means that the resistance is complete and active. Hence, females, even those who are willing to evolve by dawn stone, cannot because their bodies dictate that they can’t be affected by it, whereas their male counterparts don’t have that same defense.
The Pokédex According to Bill 