There are all sorts of good reasons for that, but it presents a problem. People have ID, but they have problems with proving their address of residence.
In some cases, driver's licences have post office box numbers on them, and everybody knows that you don't live in a post office box. That's not adequate to prove what your residential address is. They're challenged that they have to have another piece of ID.... I've seen this, where they're very frustrated. If they're a regular voter and they weren't allowed in the last election to use their voter information card as address ID, they're scrambling around, emptying their wallet, and trying to figure out how they're going to get a ballot. Their driver's licence, which they thought was going to be enough, isn't working because it has a post office box on it.