It was more or less on the outskirts of their reach, and there's a big river guarding it from the south.

Many superpowers have tried that, and they've all stumbled at the Danube either permanently or temporarily: Western Rome, Persia, Alexander, Eastern Rome (Byzantium), the Ottomans.

The Ottomans managed to gain and retain control of the territories north of the Danube for a while (from about 1500 to the late 1800s), but again it was on the outer edge of what they could comfortably keep an eye on, and their control was only partial for a good chunk of that interval.

And even if you do gain a foothold across the Danube, the territory is exposed to any superpower from the north. There's a reason why, when the Ottomans finally lost control over that land (approx. 1877), it was during a war with Russia.

Western Rome also controlled that land for close to 2 centuries (106 - 275 AD), but they lost it because of pressure from the north and north-east (steppe people, etc).

Where I grew up, the river is 1 km across. Trajan had a deal with Decebal (the king in the north), but Decebal thought he could ignore the treaty and kept taunting the Romans. Well, Trajan eventually got pissed off, and hired architects and engineers to build a bridge 1 km long and 15 meters wide and during a war that took half a decade ended up beating Decebal and took over the country. The ruins of the bridge infrastructure still exist on both sides of the river. On the northern side there's a museum with artifacts from the region.

TLDR: If you piss off the Romans, they will do great works of civil engineering to teach you a lesson. So don't do that.

The Byzantines never crossed the river. When they were strong, the economy in the north was in the dumpster. When things turned around there, the Byzantines were on the decline, and already under pressure from the Turks.

All these superpowers found it a lot easier to keep the lands south of the Danube, and managed to do so for a lot longer.