Because hexproof only helps againt your opponent's spells and abilities that target. Day of Judgment, and all other mass removal spells and effects do not target. A spell or ability only targets, if it uses the word "target" to identify what it is going to affect.

115.10. Spells and abilities can affect objects and players they don’t target. In general, those objects and
players aren’t chosen until the spell or ability resolves. See rule 608, “Resolving Spells and
Abilities.”

115.10a Just because an object or player is being affected by a spell or ability doesn’t make that
object or player a target of that spell or ability. Unless that object or player is identified by the
word “target” in the text of that spell or ability, or the rule for that keyword ability, it’s not a
target.

115.10b In particular, the word “you” in an object’s text doesn’t indicate a target.