For two reasons:
1.) No.ballast, so less weight, and
2.) Greater distance between the main lifting foils.
Also, two slim hulls are easier to push through the air than one wider one, even if the beams of the two narrow hulls add up to be the same as the wider hull.
I don't think you can make any of those assumptions. An AC75 is 6,450 kg, an AC72 is 5,900 kg, so really in terms of kg/unit length, the difference isn't huge (312 kg/m vs 268 kg/m). The new AC75s will be lighter* (maybe 800 kg or so, perhaps 273 kg/m) so the difference is reduced.
The separation of foils and windward ballast gives the AC75 enormous righting moment, plus the foils can be canted. The multis have an advantage in using the windward rudder to increase righting moment, but the rigs are very different so the comparison is moot, particularly as both are limited by class rules so not really the ultimate of either configuration. The AC50/F50 shows what happens with a more open platform and continuous development with small gains.
Lastly, both are highly developed aerodynamically, but they have very different features. The AC75 hull is somewhat more bulky, but there's only one hull and no tramp so I think again they'd be very close in drag.
But the point is moot anyway, they'll never race each other and speed around a course could be tested pretty accurately in simulators (if there's still an AC72 simulator around, perhaps an F50 simulator would do).
* Apparently the all–up weight is about 1,000 kg lighter, but that includes two fewer crew, so 800 kg less for the hull seems reasonable.
