I have no float experience so can someone tell me why he preferred the grass?
No particular reason I think; most amphibians of this type have main gear that doesn't extend very far from underneath the floats - bringing the floats into contact with the grass if it's quite long which would create an amount of friction on take off. They also have quite small wheels at the front as well that I would have thought would have a tendency to dig in on any soft surface. A hard runway would give much better take off performance.