About 4130, it designates a do-everything steel, depending on heat treatment.
Gun manufacturers use it because of its elasticity (non-brittleness) and machinability.
The Navy buys it by the ton because they can use it to fabricate almost anything, and it will assume different properties depending on the heat treatment (everything from springs to bullet proof armor).
I assume bike makers use it because the tube makers can make tubes out of it easily (cheaply), and once made into tubes, and, if not too improperly heat-treated, it tends to remain elastic rather than becoming brittle.