That's exactly it. Splash has a LOT more mechanical systems that are either submerged, partially-submerged, or are in contact the water than any other attraction. You have lifthill chains and motors, advancer belts before each lifthill, submerged emergency-stop barriers, the big flume pumps that pump water to the top of the mountain...
A ride like Small World or Pirates only has to worry about flat water with booster pumps along the way to keep the boats moving, plus a couple belts and one lift on Pirates.
And how many of those animatronics in IASW or Pirates are *right* by the water like they are in Splash?
-Rob