Materials and machining might have allowed automatic weapons to come about slightly before they actually did, but sometimes you just need a "flash" of insight to bring it all together.
But your question is still a good one - I've often thought the same thing about other developments in math and science. Sometimes, you'll have two or more people coming up with the same mathematical or scientific insights within a few years of one another, working completely independently. I think it's true that sometimes, you have "an idea whose time has come".
For that matter, why did it take so long to come up with the theory of relativity? If you look at Einstein's original 1905 paper on special relativity, it's only about twenty-five pages long, and the mathematics required to describe special relativity doesn't go much beyond high-school algebra and freshman calculus. Any scientist or mathematician from the early 1800s would have understood it, had they seen it. Einstein's paper does reference Maxwell's electromagnetics equations, but even those had been around for over 40 years.