There is a spec in the FARs for normally certificated airplanes that prop clearance needs to be certain minimum distance from the ground.
Without looking it up, I believe it's in FAR23 and the rule is 10 inches? If I'm wrong about the details, somebody please correct me.
Anyway, the gist of it is that the 74" prop potentially blows that minimum on the tailwheel airplane during landing.
Personally, I don't thing that 10" vs. 9" prop clearance is a big deal, but Vans doesn't have different performance numbers for either one, so I'm going with the 72" just because that's what they recommend.