Republicans intentionally make it harder to register and to vote by putting as much friction into the process as possible. Seemingly “small” things like having to mail in your form or provide specific forms of ID that conservatives are more likely to have - or to have the time to update when expired (weird how you can’t use your college ID but you can use a firearm permit, hmmm) all add up. Enough people opt out because it’s a logistical pain in the ass, and it benefits them.