Alt-right isn't automatically synonymous with tribal, but if you self-identify as alt-right, browse /pol/, namedrop grand wizards of the kool kids klub, wear blackface, and write stand-up routines making fun of slain black teenagers, you might just be a tribal.
So there can be an alt-right but there can't be an alt-left because.. why exactly? Does the left get special treatment? Lol
There isn't an alt-left not because there
can't be, but because nobody identifies that way and the term is meaningless. The closest thing to an alternative left right now is the Progressive movement.