The reason ac is used is that the power transmission is 3-phase. That means that , for a balanced system, i.e. each phase equally loaded, the current in the neutral wire is zero. so you have tree wires, each carrying one third of the power, and you don't need a return wire. DC systems carrying the same current would require six wires. Since weight equals cost on an aircraft, the choice is obvious.
However, dc has the advantage that multiple generators do not need to be synchronised in frequency and phase.