It is true that generally, conservatives have tended to support more strict voting laws, but I'm actually not sure if I buy a fundamental psychological explanation for liberals voting more by mail specifically, since most research I can find suggests that states introducing mail-in voting didn't really make a significant difference in election results and turnout before 2020. It seems to me like the divide we see now is because of the specific context of Trump saying mail-in ballots were illegitimate and making it an explicitly partisan issue + liberals broadly taking the pandemic more seriously (making them more likely to pick the option that you can do from home).