This is a Big History question, and thus not one that can be nailed down to a specific answer. I'd also recommend checking out /r/AskAnthropology and /r/Anthropology.

What I've picked up from Fukuyama's The Origins of Political Order and Ian Morris' books, is that technological change builds on what comes before. You don't get to build much of a foundation merely from oral history, which is all we had before the invention of writing. You don't have a class of people who need to invent writing and spread its use without agricultural surplus. You don't get that without domestication.

So perhaps that's your general answer. Concentrations of gatherers in the Hilly Flanks stumbled on the domestication of wheat. Tribal societies coalesced into states based on agricultural surplus. Writing was invented to manage the surplus while professional writers, scribes, could be supported on it. From there, we've moved in fits and starts toward collecting and passing down knowledge.

Mathematician not historian so you can delete this if it's not useful.

The rate at which technology improves is exponential not linear. Take Moore's Law for example. It says the number of transistors in a chip will double every two years.

This means you will always look back and say "wow, 50% of the progress has been made in the last 2 years and 75% of it in the last 4 years!" However the way the technology is improving is somehow the same all the time.

If all of technology works like Moore's Law then you will always look back and say most of the improvement happened in the last period. You could say the last 100 years or the last 1000 years instead. But actually the way things have been improving has been the same all along.

As other have mentioned there are plenty of mechanistic explanations such as population density and increased communication, as well as a general trend of technology to build on previous technology in a somewhat exponential manner. But I think there is a broader issue at stake, namely that there is often an implicit assumption that technological development is natural and inherently desirable. But this isn't often the case. Hunter gatherers, for example, had significantly higher standards of living than early agriculturalists--arguably the most important process in the human experience was, overwhelmingly, a rather negative one. Hunter gatherers could devise ingenious methods of resource collection, such as nets, fish traps, hunting pits etc, but they didn't invent supercomputers because, well, why would they?