Do we know why the Scythians and Sarmatians were overpowered by the Goths, Huns, and Slavs so quickly?

Question

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 the Medes and Babylonians destroy the Neo-Assyrian empire, plundering all the way to the borders of Egypt.

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Outdated tactics?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.

Answer 1

Th fight of the Sarmatians (such as the Scythians before) was based on heavy cavalry. This heavy cavalry was provided mainly by nobles, or feuds. Those feuds had territories on which their family, and sometimes their tribes, were farming, fishing, and raised cattle. Overall, there were not that much of feuds for the total number of Scythians/Sarmatians population.

This is the first liability: low number of fighters.

This system had an advantage: those fighters had great horses and equipment. But there was another problem: the Scythians/Sarmatians used heavy cavalry as a strike force and mounted archery as a skirmishing force. As the battle of Jaxartes shows, they were not open to new and inventive tactics: this was fire and strike until the enemy flees.

When the Goths came, the Sarmatians faced a numerous opponent with strong unit of all kind (infantry and cavalry): this could have been beaten tactically only with adaptation of tactics, that the Sarmatians were slow and reluctant to. On a strategic point of view, the Goths forced massive withdrawal by seizing land quickly.

Against the Hunnic army, it was worse because the Sarmatians faced similar soldiers with similar tactics, but more numerous and better tactically: the Huns massively used the retreating tactic of the Parthians.

So ultimately, the Sarmatians failed as a people. But their heavy units of cavalry were still valuable one integrated in the larger and more adaptable Roman army.

EDIT:

The tactics used by the Goths combined infantry to fix the enemy and cavalry to charge him, as they did at Andrinople against the Romans. The Sarmatians were unable to assist their heavy cavalry with other units. The usual result for such a situation is that the Sarmatian first charge is devastating, then the heavy cavalry is stuck by enemy infantry and cavalry and can't fallback to charge again with full power: noble cavalrymen are slowly killed and this results in the disaster for the total Samarthian army, that had not that much nobles to send to fight.

Goth forces did use infantry as main block on which the Sarmathian charged, but they also used, according to the source, "a supporting heavy cavalry force equipped with lance and sword.". This force allows the Goths to use advanced tactics while the Sarmathians lacked good infantry, they had only cavalry.

As a side note, as far as I know, both Gothic and Sarmathian armies had archers, but they stuck to auxiliary roles.