Postmodernism isn't really characterized by a position on which works should be read so much as how they should be read. While postmodern thinking opposes canons it also supports reading culturally relevant texts with a critical/subversive eye. Shakespeare is rich with cultural context while also being complex and ambiguous enough to provide a space for lit critics to play with meanings and interpretations and get interesting results. Hamlet, which is far and away Billy Shake's best work, is particularly conducive to this. They do the same thing with Chaucer, actually, particularly the Wife of Bath's tale. I don't think it is about style over substance but about the freedom to play with cultural meaning and interpretation. You can't say Hamlet is short on substance, anyway.

But the extent to which authors like Chaucer and Shakespeare have become less central in lit departments is almost entirely due to this crowd- it's archetypal postmodernism which gives genre films and television the same importance as the historical Western canon.

Rosencrantz and Guildenstern are Dead probably boosts the Bard's popularity in the pro-postmodern scene.