On any given day, you can earn a maximum of 200 rep from upvotes. Is there a corresponding cap on reputation you can lose from downvotes? If not, why the distinction?
The daily reputation cap exists to prevent users from gaining privileges too fast without proper site experience. Losing privileges is far less of a problem.
Besides, if a user is attracting that many downvotes (and it is not a serial vote by one other user seeking out posts to downvote; these are reverted anyway), we really want to make sure that that user is not going to do too much damage to the site. Capping reputation loss would not benefit the site as a whole in such cases.