Because guests have inflated views on what seasonal parks' opening days are when the reality is most of the staff is either newly trained or a few months out of practice, systems are getting the kinks out, and it creates a perfect storm.
Even for parks that are opening big new rides.
There are a TON of better days to go.
When I go to KI's in a few weeks, its more to get back to the park and enjoy it slowly, not to go all ride warrior (that's for mid-May or early June for me at the earliest).