There any reason as to why only the female Salandit pokey evolve and not the males?
Well according to the Pokedex for Salandit
Violet dex: Only female Salandit can produce gas laden with pheromones. Males entranced by this gas will do whatever the females tell them.
Ultra-sun dex : The males will do whatever the females tell them. They give the females most of their food. Due to malnutrition, the males can't evolve.
and for Salazzle
Sun Dex: For some reason, only females have been found. It creates a reverse harem of male Salandit that it lives with.
Sword Dex: Only female Salazzle exist. They emit a gas laden with pheromones to captivate male Salandit.
Shield Dex: The winner of competitions between Salazzle is decided by which one has the most male Salandit with it.
Violet Dex: When two Salazzle meet, they will use their pheromone gas to fight over the males in each other's group.
So basically a combination of Female Salazzle and Salandit's pheromones and Malnutrition (due to giving away their food) if the Pokedex is to be believed. At some point I hope they retcon it slightly and give it a male evolution but for now it seems to be their natural adaptation.