Coaxial rotor systems are limited by the rigidity of their rotor systems. The bigger the aircraft, the bigger the rotors, and the bigger the rotors, the more the rotor blades bend up and down at the tips. With contra rotating rotors tip divergence will cause the rotor disks to strike each other, especially under maneuver loads. That would certainly be a very bad day for the crew. The only solution is to make the rotors more rigid, which dramatically adds weight and causes other problems, or space the rotors farther apart, which dramatically increases drag. So the advantages of the coaxial configuration are cancelled out by the increasing size.
To give some perspective of how much the blade tips bend under maneuver loads, consider both the MH-47 and MH-53 which are inflight refuelable. The pilots are warned and trained to NEVER "chase the basket". Chasing the refuel basket can result in maneuver loads that cause the rotor tips to impact the helo's refuel probe, which when extended extends beyond the rotor disk. A very bad situation for the crew.
Here's a video of an H-53 crew that survived a rotor/probe impact during aerial refueling:
BTW, this was a VERY experienced HAC at the controls. It can happen to the best.