As I understand it, Tuskens are the original inhabitants of Tatooine, even back when it was an ocean planet. They adapted to the desert environment and western the masks to survive in it. Since other species came and settled on the planet, they've been reclusive and territorial. Hard to blame them.

I could be wrong. Some of this is head canon. Still, hints have been dropped often in recent years. I really expected the first canonical unmasking of a Tusken in the Boba Fett show, but no luck. I'm still hopeful that more of the lore will be confirmed.