They were more desperate and they fought differently.
When nomadic tribes migrated, they brought everything they had with them, so they had much more to lose if they were defeated. The fact that they had nowhere to go home to if they were defeated also made them even more determined.
Nomadic warriors typically fought from horseback with ranged weapons while more advanced civilizations fought mostly on foot with close combat weapons and shields. This mobility made them more difficult to hit and also meant that they could tire out their enemies by making the footmen chase them. They also were able to trick their enemies by running away for a bit to make it seem like they were retreating and then turning back and attacking.
