Due to the relative cheapness, softness, portability, and (adequate) thickness for sculpturing, the US 5-cent coin proved perfect for carving into highly-creative works that can be carried around in your pocket.
Alterations of coins had occurred in Europe, S. Africa, and the US in the 18th and 19th century -in cruder forms- but introduction of the 'Buffalo' nickel in 1913 brought about a surge in popularity of the reliefs, as the large, raised Indian on the coin allowed plenty of material to work with.
I remember as a young coin collector that the too-high date on the lower left of the Indian wore off faster than any coin, leading inevitably to dateless nickels- the buffalo nickel design was flawed from that standpoint. But the fat Indian relief was what made it so ideal for hobo nickels.
The 'hobo nickel' term itself is generic, as they've come to include all denominations of coins, US and otherwise. The art form enjoyed a significant renaissance in the 1980s in the wake of a book published on the topic by a numismatics expert... thus the modern variants: