Great point that's often over-looked..
In early 1943 the US Navy considered both the F4U and F6F to be excellent carrier-based fighters but during this period the US Marines desperately needed newer fighters in the Solomon Islands to replace their worn-out F4Fs. With Grumman ceasing production of the F4F in favor of newer more advanced F6F, the ready available F4U seemed like the best candidate as the Wildcat's replacement within US Marine air units.
The F6F was just becoming operational with the US Navy during this time. With it's more docile handling characteristics it was thought that carrier pilots could be commissioned at an even faster rate so the US Navy decided to continue equipping US Marine air units with the F4U and made the F6F the standard shipboard fighter.
This makes perfect sense, as the supply chain was already fully established for the F4U so why change anything when you have Grumman solely concentrated on F6F production to fill the need for future shipboard fighters? It was the simplest answer which allowed for both requirements to be met in the shortest logistical time-frame possible.