I'm doing a project on the Scythians and Sarmatians and so I've been reading up on ancient wars involving them and I've noticed the following:
So, from around 500 BCE to 1 CE the Scytho-Sarmatians were on top of the world.
They defeated and killed Cyrus the Great and sort of defeated Darius.
Then later on they lost a battle against Alexander, before destroying a Macedonian force of 30,000 under Zopyrion and joining Alexander's army against the Indian king Porus. Even scoring some wins against the Romans.
But then it all seems to have fallen apart.
The Sarmatians (functionally Scythians in terms of culture and warfare) performed miserably against the Goths in the Gothic-Sarmatian war, then were immediately afterwards conquered by the Huns. After this, the Sarmatians get conquered and completely assimilated by the Slavs. All of that within about 300~ ish years.
So what gives? How did the primarily infantry armies of the Slavs and Goths overwhelm the Scytho-Sarmatians so easily? What did the Huns do that helped them triumph over the Sarmatians?
Was it economic decline? This seems to have been the case among the Iazyges tribe for a bit, but that couldn't have been the case for all Sarmatians and Scythians.
Inferior leadership? The Scythians and Sarmatians consistently were not a united people after all.
Outdated tactics? Of course after years of hegemony you're going to slip up every now and then, and enemies can easily exploit that.
Were they simply worse warriors? I doubt that the military capabilities were simply worse than their enemies, but I guess it could be possible.
What happened to them that caused them to be destroyed so quickly and profoundly? Where did the Scytho-Sarmatians go wrong? Do we even have any idea?
To be clear, what I'm looking for here is analysis and insight on Scytho-Sarmatian warfare vs Hunnic, Gothic, and Early Slavic warfare and how the Sarmatians might have been so utterly defeated. I'm also looking for occasional literary answers on the subject if that can be provided, because I cannot find any.