Higher voltages and frequencies is how you get less loses as resistance is coupled to current. The lower the current, the less the resistance affects transmission. However you can only increase ac voltage and/or frequency so much before you have another issue, the impedence of the air itself starts to create loses Also, because AC doesn't fully penetrate the wire, you have to run much larger wire sizes to achieve the same affect. At a certain point, you simply can not push more power using AC without resulting to things like superconductors.

DC OTOH, does not have the skin effect issue and so because more desirable in certain cases. In fact, high voltage DC is how they electricity directly to LA all the way from Oregon/Washington state: https://en.wikipedia.org/wiki/Pacific_DC_Intertie