Keep in mind when avionics started, we Had to have AC to make the avionics work. Synchros and resolvers need AC in most applications. These are the gyzmos that drove the compass cards pointers and such in the steam gauges. And the tube type radios were able to me made much lighter with an AC power supply that could be easily stepped up and down in voltage.
Some aircraft like the newer King Airs with the proline 21 avionics are now DC only, getting rid of the weight associated with the inverters.