Honestly, this is one of the things that I found always weird. Like you want to tell me that every Werewolf and Gigant/Half-Gigant out there is just plain evil and/or brainwashed except Remus and Hagrid? Yeah… I don’t buy it!
There must have been something that stopped them from wanting to join. Was it–and that is just a thought–mayhaps the fact that nothing would have changed for them on the light side? That there was no incentive for them to join, that they would have still been treated as a threat and “dark” creatures regardless of their intelligence. Voldemort at least promised them change, it doesn’t matter if that was a lie or not. Or maybe it was because there was someone so unbecoming on Dumbledore’s side that they were just driven off? I, personally, believe it to be the former.
Think about the treatment of Goblins, they got their bank, but they are still not allowed to use wands, they aren’t equal. There must have been a docent other children just like Remus but the only Werewolf we know about that attended Hogwarts was well… Lupin and the same goes for Hagrid. We hear about how hard it can be for them to get jobs or hold them for that matter. (Though sometimes that is deserved, looking at the Werewolf incident in Harry’s third year 👀.) For crying out loud, they are categorized as dark creatures, not human, not even as just creatures, no, as dark, as evil their existence is seemingly seen as harmful.
On one hand, I am barely more than a dark creature, not equal to wizards, sometimes not even thought of as sentient, on the other hand, I might get more rights and maybe even equality. I don’t know about you, but if I were a “dark creature”, I would join the dark side. They at least give me a chance!