Vectors, the power distribution in airliners is 115v / 400Hz AC because in that format you can deliver much more power over a smaller, lighter conductor than you can at 12 or 24 volts DC. Filtering out that AC component for the B+ supply of the final amp in the VHF transmitter required comparatively heavy chokes and capacitors not needed in a DC powered environment, and the increase in weight beyond a certain point just to completely get rid of that AC modulation component reaches a point of diminishing returns. Newer avionics are better at filtering out that hum because they employ solid-state switching technology (like in a PC computer power supply) to convert the AC to required DC levels, so you don't hear it as often as with the older aircraft. In fact I would not be surprised if they use pulse-width modulation (PWM) in the final amplifier stages of the transmitters that is powered directly by the 3-phase AC.
I personally believe that for many years avionics manufacturers could have easily filtered that 400hz tone out, but because turbine aircraft were the first to employ those power systems the characteristic hum in the transmissions became a status symbol among pilots. The moment you heard it you knew it was one of the "big boys", Leo DiCaprio in his Pan Am captain's uniform just back from Paris, and they liked it that way. In fact I would not be surprised if they still have an installation option via firmware settings where they can actually inject the missing 400hz tone back in again if pilot ego demands.
It reminds me of a story my dad told me about the old Bell system, he being a Bell of PA engineer for his entire career. When they first got the Long Lines microwave relays up and running they had a problem with the audio quality... the signal to noise ratio was so good that people would hang up before allowing long distance connections to complete, erroneously thinking the line was "dead". So, the solution was to actually inject a small amount of white noise at each stage so people could "hear" the connection being completed, and as an old timer you may recall those cross country calls where you would hear things become more "distant" in three or four steps prior to the ring commencing. What few would notice is that usually after the other party answered that "connection" noise would go away.