Er, sorry, Koaster King, but Son of Beast was NOT completely built on opening day last season. Some of us who were there that day have some interesting photos of incomplete bits of the ride.
That said, Kings Island's season starts extremely early in the spring, and ends very late in the fall. The short off-season is often combined with bad weather. They frequently schedule their new attractions to be open in mid- to late-spring rather than at the beginning of the season. Sonny is the only ride in recent memory I can think of that was actually significantly late. Sure, Ghoster Coaster, Outer LIMits and Drop Zone were all late, but they were also announced for 'late spring.'
--Dave Althoff, Jr