Salafist background. The Quran and Suras says that Mohammed wore his beard long, with a shaved upper lip. The books also say everyone’s hair should be one length, that’s why you see guys either with a shaggy Zabit look or with a shaved head a la Khabib.
The younger guys grew up in the wake of the first and second Chechen wars. The insurgents embraced Islam heavily after fighting the Russians. It’s heavily ingrained into the culture.
The Dagestani Avars, Dargins, and Chechens of Khabib’s father’s generation were old school soviets. Religion was obviously outlawed.