This comes from typewriters. Curiously, the reason why Esperanto uses Ĉ, Ĝ, Ĥ, Ĵ, and Ŝ is because the circumflex was present on French typewriters (which were very common in Europe at the time). Even though French itself only uses it for Â, Ê, Û - since it was a distinct key used for overtyping, it could be repurposed in this manner, just like Unicode combining marks today.