I think a more appropriate statement would be, why do neo-conservative Republicans bother. Ron Paul is essentially libertarian/classical liberal (a political stance which I think is widely misunderstood, due to a two-party mindset). He's running Republican because its nearly impossible to get on all the ballots as a 3rd party candidate, unless you are extremely wealthy.
Perhaps because modern Democrats tend to be socialist and modern Republicans tend to be corporatist. Did you know that Bill Clinton set the stage for regime change in Iraq?
Actually, in following Ron Paul's progress, I've seen unusual online support for him from Canada, Australia and Europe.