Innovation/Invention requires a lot of trial and error and the ability to build on previous results. Until a few thousand years ago, these experiments were local, and there was little we could learn from others' experiments. Thus, a guy in Ethiopia might have been trying to master fire control even 5,000 years after a guy in Sweden has already mastered it. There was no easy way to transfer ideas given the lack of wheel (to enable quick movement), writing systems, broadcast communication, etc. The population was also too low to improve the odds of experimentation.
Lastly, we were too focused on survival to afford us the time to innovate. Agriculture liberated us from the focus on the daily search for food. Finally, we are constantly discovering more about our past, and our knowledge of our ancestors is not complete. A hundred years ago, we didn't know about the magnificent Indus Valley civilization and knew little of Mesopotamia or Incas. New discoveries are constantly pushing back the known history, and I would not be surprised if we discover more complex civilizations from 10000 BC that have just been lost due to the passage of time.The answers are obvious: compounding, increasing population and communication technologies, etc. I would also recommend reading the first part about compounding with an eye to why every country is in debt right now.This is also the justification used (in my opinion, incorrectly) to argue for the "singularity" - that technological progress will become infinite.
I'd take issue with a couple of things in the second answer, however (besides the value judgement implied by the term 'progress'). #4: There is evidence for human use of fire for over 1 million years. The debate rages (like a fire) as to whether human fire use was intentional or opportunistic, but I think Richard Wrangham's hypothesis is compelling. 3# is horribly overstated. Apparently, the writer didn't bother to read his own link, which states that life expectancy of adults (after surviving childhood diseases) was 54 in the Upper Paleolithic.
I'd also point out that we've had to innovate ever faster, because we kept causing problems with earlier innovations. Agricuture led to increasing populations, which led to irrigation and the need for sanitation, etc. Increasing populations led to more wars leading to military arms races, etc.
See also: Lost Civilizations of the Stone Age