I was involved in a fairly intense discussion over this a few years back with some other people in the business, when Boeing first floated the jet as a civil lifter.
Firstly, it's got excellent short field characteristics. That doesn't always sit with excellent economics. It's got four 757-type engines hanging off the wings and a fairly small cargo volume if you compare it to another four engined hauler like the 747 or An-124. It would be useful in one particular industry, hauling heavy gear around for the oil business, but what else would it relate to?
It could also make money in the engine ferry field, where at the moment a mix of Antonovs and other kit haul them around at short notice.
That's the main issue, block hour cost. It's designed for versatility and not economy. Is there enough fringe business in freight to justify the high capital cost and high running cost of a fleet of 30 or more?