Why did FedEx Express order B767F instead of A330F? Do their B767 usually fly well below MTOW, but with more freight than a narrowbody can handle?
Wingspan is the issue with the A330. You can't look at the integrators really like you look at any other airline. Almost their entire fleet is on the ground is on the ground during the sort. Yes there is a stagger to arrivals and departures, but there will be a point in the night where every airplane you intend to fly out at the end of the sort are on the ground. This means maximizing your space is key.
Look at it like a math equation. Let's say you have 100 A330s on the ground at that midpoint of your sort, which is the max you can fit. If you made those 767s you can fit 126 in the same area (Really it's less than that due to wingtip clearance requirements, but it was more to prove a point). The A330's wingspan is that much greater.
It's an even bigger problem at the destination. There are a lot of places that are set up for a 767 sized wingspan. In those places you may have 3 767s on the ground, but you could only have 2 A330s. And the A330 only carries about 1000 cubic feet more, so you're not making up the difference. Any gateway that has enough to send an A330 to will have multiple flights on the ground at once, so it's critical everything fits.
As for the widebody vs narrowbody debate. For domestic it is going to come down to cube size, very rarely weight. Air packages are usually lighter. Not many people are ordering refrigerators next day. There are plenty of occasions where a 757 weight wise could probably carry what is on the 767, but because we're talking about light boxes that take up a lot of space, the cube makes the 767 necessary.
Look at sweatpants guy. This is a 90 million dollar aircraft, not a Tallahassee strip club