Idk. I think the real tipping point is being able to succesfully use external memory so that you don't have to keep everything in your working memory when doing reasoning or solving problems.
It was just most probably a nurture thing — there was not as much accumulated knowledge to “bootstrap” a more advanced civilization (or perhaps, no will to do so. Why change when your system works just fine?). It’s not like Einstein could have achieved much back then without “standing on the shoulder of giants” before him.