> Bauhaus did emphasise function at all costs
That is not true. They went for a new blend of usefulness, looks and manufacturability. The look was just new and different (although similar to some precursors: De Stijl and some Russian constructivism), it wasn't ignored at all. Today, many things seem to be cargo-culted Bauhaus designs with many of the good aspects removed. Not all buildings with "simple" shapes look good.
Cost. You don't have to pay for and manage pesky craftsmen with their finicky details. Fabricate some concrete slabs in a factory, ship them to site and hoist them into place and you're done.
At the very least the art-deco guys tried to find some compromise with form, and Bauhaus did emphasise function at all costs.
Brutalism for its own sake reminds me of the dudes who hear the trope "nice guys finish last" and then go about acting the scoundrel just cause they think the opposite might be true. Maybe they get a few "wins" here and there but romantically, and to all practical intents and purposes it's a dead end.