When the Germans projected the A4 missile, they discarded solid-fuel propulsion and went on to circumvent the enormous technical difficulties of building a liquid-fuel engine from scratch. Germany had, at the time, perhaps the best chemists in the world, but they were unable to formulate a good mixture for a solid rocket fuel. Is the present abundance of large solid-fuel rockets due to some recent discovery...?
Contrary to what we learned in school, solid motors are not "dumb and simple". They are very, very hard to put together. Any small irregularity in the propellant grain can lead to a hot spot and failure of the pressure vessel.
To get an idea of the massive machines used for building the SLS motors take a look at this video:
I've seen the actual mixers. Imagine your standard Kitchen Aid mixer, but with a bowl the size of a small room! It's not just about the chemistry. Is the entire process.