One of the advantages for medium sized aircraft is that the DC generator is also a starter. No additional starter motor required, and you can start the engine directly from batteries. It is also easier to step down DC voltage if required (e.g. 28VDC to 5VDC).
If the generators were AC type, it would mean that a CSD should be fitted as well in order to provide stable 115VAC at 400Hz, despite power/turbine speed changes.