I couldn't find anything on why specifically Mandibuzz are female, and I thought both KitkåtKK2™'s and PrimalKyogre's theories made sense. Also in Mandibuzz's Sun entry, it mentions that they are all female, and they wear bones to attract males.
On the other hand, (while it still doesn't make complete sense) I did have a theory on Salazzle. Female lizards tend to be larger than male individuals, which could be a possible reason why only female Salandit evolve, to define the fact, that yes, they are female and in control. But lizards don't usually live in groups, unlike what its Pokedex entries suggest.
But, I did find a really interesting theory on Youtube discussing maybe why only females evolve, here.
This stuff is really confusing though, so you can go ahead and blame it on Game Freak logic. I won't judge.