Does String Theory Satisfy Einstein's Conditions for a Physical Theory?
Actually the original string hypothesis was very simple with gratifying results. You just assumed the fundamental "thing" was a vibrating relativistic string, and took the simplest action for such a thing and developed it, and you got a graviton that was consistent with Einstein's field equations (so they thought and many still do). You got several other particles, including a tachyon you very much didn't want, but there was that graviton, and there was no 'fine tuning' or 'epicycles' about it.
The trouble was they never really got better than that first breakthrough, instead they got deeper and deeper into the math of their models and more and more contingent.