The process is intimidating and confusing to a lot of people. Most states require you to pick a party when you register and many people don’t want to pick a party not realizing they can vote however they want to later. They don’t know who the candidates are or who supports what issue. Not everyone is politically engaged.
Political parties like to put people into buckets and of you’re from a certain area or demographic, then they don’t want you to vote or they already assume they have your vote or that more votes don’t matter.