I feel like I can’t find a good answer to this anywhere that I look. It seems as though it basically just goes that through all of history mounted steppe warriors were decidedly superior to those of settled communities aside from a few rare occasions, and then all of the sudden mounted steppe warriors can’t do a thing against the communities around them around the 14th/15th centuries after battles like Kulikovo and the like.
What military or technological innovations occurred?
We had a similar question last week, and I wrote a couple brief comments with a link to an earlier answer here.
Just to expand on it a little: if we're talking about settled peoples really subjugating steppe peoples militarily in any serious, permanent way, then we're mostly talking about the Russian Empire and Qing Empire in the 18th and 19th century.
I'm much more familiar with the Russian end of things, and basically what this involved was first building a network of reasonably well defended forts along the border in the 18th, as well as establishing protectorates and clients among steppe peoples beyond that border (as border regions were secured, this process could be repeated further out). But full-scale conquest, as in really defeating any military threats, was more a product of the 19th century. Rifles, machine guns, the telegraph and railroads weren't initially part of that mix, but eventually would be.
More can of course be said, but you may find these two answers that I wrote a while back to be of reasonable interest:
Hi there! I've covered your question partly from the other angle -- i.e. why did a nomadic archer military tradition die in the period you describe? This is a Eurocentric answer and has nothing to say about the various Chinese dynasties or any other similar non-western examples as they are beyond my area.
The thread itself is better than my answer in particular, but for further reading I would really recommend Geoffrey Parker's The military revolution: military innovation and the rise of the west, 1500-1800 which is based on Murphey's work in the 50's which attempts to delineate the early modern period from the medieval mode of war. In my biased view one can see this best in the evolution of the Ottoman military system and it's response generated by the Habsburgs and later the Russians.