Why is this considered a reasonable factor that's often brought up in discussions rather than other somewhat unique aspects of their government and society?
Because making it possible for ethnically/racially/culturally diverse groups to live happily together is and has historically been a major challenge. So a country that has immigration policies that promote homogeneity will avoid many of those challenges. It's certainly not the only factor...but I can't imagine that it doesn't play a major role.
I gave it 4 days and I still think it's a rhetorical Trojan horse for white nationalism. I've quite literally never seen that argument trotted out in any other context.