This is another of those questions that only Game Freak can answer, but I will state a theory.
You're describing r/K selection.
r selected organisms have harsh selection pressures, and need to mature quickly to reproduce.
K selected organisms have fewer predators and dangers, so the advantages afforded by maturing more slowly (probably greater brain development, among others) survive with more frequency.
Think about it. We'll use your examples of Pokémon. Mienfoo is based off a weasel type animal, and its natural predator is probably bird of prey(Braviary). Even so, Mienfoo is a Pokémon that is experienced at combat, shown by the stance and design of the Pokémon, and as a result can probably defend itself to an extent.
Pawniard is protected by steel, which means that it is not a prime option for a meal, and fighting type Pokémon aren't going to eat it, or just randomly decide to beat it up(hopefully Pokémon aren't that violent).
Vullaby is more or less an adolescent chick, and is probably high in a nest up on a cliff face or tree, being a buzzard chick. Its main predator is birds of prey, which come from above. Adult buzzards aren't small birds though, and could defend their nest against predators if need be, and Mandibuzz has a good Defense stat, the main bird of prey being Braviary, this is fitting.
Rufflet is also a chick, and above ground in a nest, with ok real predators, perhaps Mandibuzz if the nest is left unguarded, but Buzzards are mainly scavengers.
Deino is a dragon, and thus doesn't have many predators.
Larvesta is a larvae, meaning it spends most of its time underground, so its main predator would be Excadrill, and the chances of happening across one are quite slim.
So basically, unlike other regions, many of Unova's Pokémon have less predators, and survive longer, so there is no need to evolve so early. Bug types are generally weak, and have many predators so they need to evolve fast to survive and reproduce.
This is just a theory, and it may be a completely different reason, only Game Freak can tell you the true answer.