I don't entirely understand--Alexander's first campaign included an expedition across the Danube to subjugate the Getae, who were inciting revolt among the Thracians. The Antigonid and Lysimachid kingdoms both expanded far to the north, towards the Danube and sometimes crossing over

There really wasn't much that anybody wanted up there near the Danube at the time and unlike under the Romans the local populations were poorly organized and not much of a threat--indeed a major, perhaps the major factor in the Dacian wars was the presence of an increasingly unified and organized hostile group along the Danube. Additionally the Greeks really didn't know much about the Danube, certainly not in comparison with the east. And besides, the territorial worth (not to mention the vast wealth that could be made in tribute and plunder, along with legitimate trade) of the east was many times greater than anything that could be exacted from up in the Danube basin. It was enough for the Greeks, and for most of the history of the Romans in the Macedonian region, to keep the inhabitants from raiding and causing trouble