AC runs into a problem with skin effect. At a certain point, making conductors thicker won't make them handle more current. DC doesn't have this problem.
Why AC? For a given power transmission line, higher voltage will lower transmission loss. Historically, it was easier, cheaper, and more efficient to get higher voltages with AC using transformers. These days, DC is still more complicated and expensive than AC, but the efficiency has gone up and the price has gone down somewhat on the DC side of things. And if grid synchronization is a problem and you're transmitting a lot of power, the benefits might outweigh the costs.
Why AC? For a given power transmission line, higher voltage will lower transmission loss. Historically, it was easier, cheaper, and more efficient to get higher voltages with AC using transformers. These days, DC is still more complicated and expensive than AC, but the efficiency has gone up and the price has gone down somewhat on the DC side of things. And if grid synchronization is a problem and you're transmitting a lot of power, the benefits might outweigh the costs.