possibly because these wheel pilots never worked very well for heavy displacement boats and RayMarine decided wisely to reduce the rating. This way they get less complaints from owners that decided to buy cheapest possible system for their boat, and at the same time get to sell more expensive systems instead
Win-win for them.
Seriously, though - wheel pilot systems cost about as much as linear drive, and a cost of building a hydraulic system is higher but certainly not so much to make it prohibitive. The result would be far better with either of the latter.