Some of the super high voltage transmission lines are DC now. To bring DC to the home requires highly efficient DC-DC converters, however. In the beginning, transformers were a very nice way to transform voltages up and down. To do this with DC requires high voltage semidonductors.
Ther used to be a dc link between england and france to make best use of nuclear plants i think.
In some cases it is difficult to ensure the synchronization frequency and phase in networks.
It is easier to rectify the AC voltage, transport it as DC and and recreate AC after transport.
In all continental Europe the electrical network is fully synchronized but apparently it is more difficult with some islands even if the frequency is the same.