Because he isn't a supporter of the military industrial complex, and he wants to legalize drugs. That alienates the neocons and the social conservatives. What's left is the economic conservatives, but these are also the business interests who want to buy government influence. That basically leaves his constituency with the principled libertarians and the people who just hate all things government.
Even though I am extremely liberal, Paul would not be in bed with corporations the way the current GOP is. He makes everyone upset!