It requires you to take time to register, either out of your days beforehand to be registered day of; or time out of your day to register to vote at the polling place day-of. This is on top of having to drive potentially a ways to get to the polling place, only to be told you cannot vote because you're unregistered.

It should be a national holiday, or you should be able to vote online. Session IDs would go a long way in preventing voter fraud, I think.