According to this site, Erasmus foolishly did not patent his design, so someone stole credit decades later:

This particular technology was first introduced in 1758 by Erasmus Darwin, [grand]father of Charles Darwin, in a paper entitled “Erasmus Darwin’s improved design for steering carriages—and cars”. It was never patented though until 1817 when Rudolph Ackermann patented it in London, and that’s the name that stuck.