This is actually an option built into the software we use. I decided to turn it off when I first made the site because I thought it made more sense to read the whole question before voting, like Fizz said. I think it also saved some space in the original design.
Although for most questions you get the gist just from the title, if you're actually interested in the question you're likely to click through anyway to read more, and read the answers. So that's the logic anyway, but I'm happy to hear more arguments for having the voting on the main page.
