Why did it take humanity thousands of years to advance in technology?

Well, the Bubonic Plague, for one thing, severely hindered progress by wiping out a vast population in Europe. Wars didn't really help, either, whether by political or religious means, and terrible communication with other countries provided little means to have the geniuses of the world effectively communicate their ideas and collaborate. When cultures that speak a unique language die out, they take their language with them. This is the case for alchemists, who had their own forms of notations, even though their total known contributions were enough to make way for Chemistry to succeed it.

Fear was also a big part of it. Roger Bacon was a man who experimented with steam power, and was far ahead of his time. Unfortunately, the less articulate would drop by, not welcomed in the least, and scurry back with tales of "the devil screaming from its gates." It can be imagined how hard it would be to actually know things.

Technology will continue to develop slower than it should, especially since it is practically legal to restrict access to it, as with closed-source software or classified test technologies by governments, or simply refusing to share a certain man's findings (we're looking at you, Russia!)

A vital question is: "What is technology?"

Many early civilisations showed evidence of the most amazing technology. Several thousand years later, the modern world looks back at their technology and says, "How did they do that?" How did they build the pyramids? (Recent evidence shows it was not slave labour.) How did Noah build the ark? (Bear in mind that Noah's ark is a story common to many ancient civilisations, and not just found in The Bible) What is the origin and purpose of the simple machines that have been uncovered? The Antikythera Mechanism, for example, continues to confound modern man. And little is known about the secrets that enabled the ancient Egyptians to build the Pharos lighthouse of Alexandria.

It would seem that humanity had technology, but that it was a simple and far more effective technology. There was no need for all the trappings that we now consider technology.

Perhaps the question that should be asked is: why do we now need such elaborate technology to live? Why have we now lost the ability to make simple but effective devices?

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Not looking for a philosophical, existential answer, but point taken. Seems to me that the great American experiment became the fertile ground that finally broke it all open.