Reagan didn’t announce until November.
Sarah is no Reagan. She doesn't have the experienced campaign staff, the established fund-raising machine or nearly the public support that Reagan had. Prior to Reagan announcing, he was already in the lead for the GOP nomination in the polls, was well known, and was generally well-liked by GOP voters. Sarah is well known, but more than half of GOP voters say they don't want her to run in several recent polls. She would have a lot of made-up minds to change, and she is running out of time to do it.
Election cycles were different then than now.