I'm on the side of ideology or "principle." People are more attached to their opinions, or being right, than they are to their lives. The "it won't happen to me" mentality, combined with "gov't is trying to ruin my life" is another. But there is also a large subset for whom getting a vaccine is simply inconvenient, i.e. taking time off work, or taking a day off work to recover from the effects, or having to drive several miles out of the way, or having to schedule something, or having no one to watch the kids, etc.