Michael Ballaban writing for the Gawker remnant Jalopnik:
The K-Max actually went through an initial production run from 1991 to 2003, and the main reason for the weird rotor configuration is that there’s no need for a tail rotor, which saps power that could instead be used for generating vertical lift. Having two main rotors which spin in opposite directions cancels out the need for a tail rotor to push against the torque of one big main rotor, much like you’d see on another heavy-lifting helicopter, the CH-47 Chinook.
But the Chinook isn’t designed specifically as a heavy lifting machine. It’s a huge, multi-purpose helicopter designed for a variety of missions, which helps explain its fore-and aft rotor layout. The K-Max, on the other hand, is designed specifically to carry loads slung underneath it via a long cable, and that necessitates it being small, narrow, and, well, weird,