When you can waste it on a doubly-complex transmission system, coaxial drive shafts that bind under side loads, and double the number of rotor blades.
Coaxial helicopters have their advantages, such as no retreating blade stall at high speeds, but also have their drawbacks as well, mainly limited total shaft horsepower that can be handled, and the increased weight of a dual rotor mast and transmission system over an equivalent conventional main and tail rotor layout.
Always a trade of course. The lift component addition vs. the apparent waste in a vector not desirable seems attractive but those who use tail rotors are not fools either. Nothing is free is it?
Kamov specializes in building co-axial rotor helicopters. All their helicopters use this system.
No other company does that.