I believe before the stirrup (as we know it) there was a simpler device that was just a strap around the horse's midriff that the rider hooked his toes into.
As warfare changed the need and use of the stirrup evolved from the simple "toe strap" for balance of the horse archer /light spearman to the stirrup in conjunction with the highbacked cantel/pommel of the saddle to help absorb the shock of the heavy couched lance and keep the rider from being unhorsed. Like most inventions, I'm sure there was a lot of trial and error. Also, if you look at most warfare inventions through the ages, there's a relunctance to accept new ways initially. For example, during the American Civil War the use of the repeating lever action rifle and Gatling gun was repressed by those in charge of the Union armories. They felt their soldiers would waste ammunition by firing too rapidly. I don't think it would be too far a stretch to imagine that using the stirrup, at first, was considered "less manly" or some such nonsense.