It is not JUST steel. The process or i would like to call it as evolution of reinforcement is something worth understanding. From the first reinforcement being just iron rod to Mild Steel and then the HYSD processing of steel bars to give it higher tensile strength, the innovation involved is extra ordinary. The current TMT technology has taken from just steel to another step forward, by imparting the necessary qualities of construction - Tensile strength and Ductility.
This link explains an alternative to steel - https://www.researchgate.net/post/Which_materials_have_been_used_in_the_place_of_steel_for_reinforcement
But one major factor for choosing steel as a reinforcement is that the coefficient of expansion of steel is similar to that of concrete. For more read on this quora link - https://www.quora.com/What-type-of-steel-should-be-used-for-construction-and-why-is-it-only-steel-not-other-metals-or-materials