>Because many of them don't have such an ID (turns out about 10% of PA voters have neither a drivers license nor the state-provided non-drivers photo ID). Getting such an ID can be expensive (both in actual costs and in work hours lost), difficult and time-consuming.

That's not true, it takes about 15-30 minutes if you go early in the morning. there are even programs available to help the indigent pay for it. There is no excuse for not having an ID.... My grandmother died at 95 and had a driver's license.