It's a complicated story but it basically comes down to efficiency; if DC were the standard, electrical power would probably cost at least 10x what it does now, and that would filter into every product that uses electricity as an input. Basically, we can't afford it, and the entire electrical system would have to be torn down and rebuilt from the ground up. This would be a century long project at this point.

The wiring and switching equipment has to be a lot beefier to handle the continuous DC current. With AC, the current flowing through the circuit is zero 6 times per second, which means wear and tear on switches due to arching is far less.