Why did general sherman march to the south to the sea?

Answer 1

Having failed to destroy the Army of Tennessee in Atlanta, he was now being ordered by Grant to pursue them into the mountains.

Sherman thought he could end the war quicker by destroying the civilian infrastructure that supported the Confederate armies in the field, and decided on a major punitive raid through Georgia, burning farms and tearing up railroads.

When he reached the sea, he would be able to relieve Savannah, one of the blockade-runners' favourite ports.

Answer 2

Because his supply-line was too long and vulnerable to support a campaign in the mountains. If he crossed Georgia, he could forget his supply-line and live off the land.