I don't follow your logic. If Kentucky Kingdom wanted to keep people in Louisville, they would open earlier and entice the people visiting the first two weeks to upgrade to season passes. They park shouldn't be trying to keep people away from Holiday World the weekend of April 25th, they should focus on keeping more people in Louisville throughout the season.
The change in opening dates is attributable to either when operations can have everything running or a cost analysis showing that the low expected attendance the weekend of April 11th and 18th will not cover the operational costs. Parks lose money when there are not enough guests.