Sadly because people cheat - so this is one of the methods to stop that. Same IP cannot vote on the same server more than once pr day. Same goes for same username.
Could only wish we lived in a better world, but any small exploitation way, people take and use. (we did try allow same ip to vote, but got misused right away - making the voting useless).
So if you want to vote from all 3, look into using a proxy service or vote from other location.