Blame the lawyers:
Why would it be harder to write a regulation in plain language than in complex jargon? In part, points out Braley, it’s because of the job a regulation needs to do: taking a law that Congress has passed and “dealing with the fine-tuning . . . putting into words how certain conduct is supposed to be governed.” That means a regulation carries a heavier burden than other writing that government agencies produce, like forms and letters. “The lawyers get their hands on them and want to make sure that they’re absolutely foolproof as far as going to court is concerned,” said Cheek.
Nice piece by Leon Neyfakh in the Boston Sunday Globe on the attempt to bring plain English to government regulations.