I don't think all states have online registration.
I'm positive not all states have mail-in ballots.
I agree with you about voter registration. It's not that difficult in most states.
However, there are studies that indicate when early voting sites or hours are reduced, or are set up only in suburbs, that voting by some subgroups is diminished. Same with how election day polling sites are set up.
Politicians should not go out of their way to selectively prevent people from voting.