Why is the name hobo hobo?

Originally the expression was 'Ho, beau...', meaning 'Hello, handsome...'

There are a variety of possibilities besides the one mentioned above:

"Ho, boy!", a greeting used among railroad worked in the late 1800's.

"hawbuck" and "hawbaw," meaning "an unmannerly lout", English dialect terms

Hobo for Hoboken, NJ, a railroad hub in the 1900's

Hobo for "hopping boxcars"

"hoe boy", an itinerant farm worker, especially during the Great Depression of the 1930's