For the same reason wy Cars shifted from DYNAMO's to Alternators,
Alternators are far more efficient than Dynamos.How ever it does depend upon the electrical design of the airplane.
For the B737 the engines drive Electrical AC Generators which in-tuurn power the electrical system and DC is generated via Rectifiers...
How ever on the ATR the Engines have DC generators which use the inverters to create AC (Constant frequency) and the Props drive Variable frequency AC alternators.
Also another reason for AC is Higher the frequency smaller is the design shape/size/weight of the electrical system/component.