Because AC is easy and cheap to transform from one voltage to another using transformers. High voltage means less amperage for the same power, meaning you can do with thinner wires. But high voltage is difficult and dangerous to use. With AC, it's easy to transform 400kV from transmission lines into the 110 or 220V that you can use at home.

with new semiconductors that are now available we can - similar to switching power supplies - convert DC as well somewhat more economically. So high voltage DC transmission lines are now becoming more popular. DC has the advantage that you have no losses due to EM radiation. So the old rules are changing due to technology.

I cannot understand this: "High voltage means less amperage for the same power" Please elaborate a little