Barkley didnt played a lick of defense, therefore his advantage as a bit better go to guy doesnt give your team any advantage if you get outscored due to no defense
A lot of people miss this point for some reason, basketball isnt just about scoring (and Malone scored more as well, in Playoffs too), and Mailman was considerably better defensive player. He actually played both sides of the floor, while Charles was just coasting on D, saving all his energy for offense, and still
he didnt reached Karl's scoring volume.
Speaking about Playoffs, Charles had better teammates overall, and yet he had less Playoffs success than Malone. No advantage for Barkley here either.
And lets not even touch work ethics... one treats his body like a temple, and another like a bag you can throw all junk food in.
No GM would pick Charles over Mailman, simple as that.