Because his parents picked it?

All joking aside, there's a precedence for people to name their kids after famous and powerful people. The Tecumseh that's most famous in the US is the Shawnee "Prophet" who tried to build a native american nation in the North East US (supported by the British). Through the early 1800's his troops battled with the US Army and settlers in Ohio, only finally being killed in 1813 during the War of 1812.

General Sherman was born only 7 years later, so memory of his name sake would be fresh on people's minds. Sherman himself mentions that his father had evidently been taken with a fascination with the Shawnee leader, and named his son after the man.

Lewis, a historian writing in the 1930's, however tells us that Tecumseh was originally the general's first name, and that his foster parents changed it to William when he was around nine or ten years old.

The accuracy of this however is in question because Sherman himself claims that his baptizing minister can back up his being named William from the start.

Sources: Memoirs, By General Sherman Sherman: A fighting Prophet, By Lewis