Basically, the answer is for 3 big reasons:
Because alternating current is easier to:
- Generate.
- Convert or transform.
- Transport.
Why AC is easier to generate? Because is cheaper and simple create big Alternator than big Dynamos.
Why AC is easier to convert? Because we can use transformers and raise the voltage. While in DC we can't use transformers (they don't work in DC).
While more voltage, more distance the current can travel.