Not trying to oversimplify the appeal, but I personally found their juxtaposition between the absurd and highly intelligent social commentary endlessly entertaining because deep down it felt like they were saying something is really really wrong with society and existence in general. They wrapped it up in comedy, because what else can you do in the face of an existential crisis but laugh at it? They always felt like a group of very angry young men trying to disrupt the stuffy old system from within. Very subversive stuff, especially for it's time. It really resonated with me as an angry/angsty (and relatively intelligent) adolescent male.

Go watch Brazil and tell me that Gilliam isn't furious with humanity and the state of things in the world.