With the exception of mineral-rich Dacia, which had mines that easily covered the expense of holding it, there hadn't been a serious attempt to conquer more central European land since Augustus (Tiberius briefly thought about it)
My understanding is that Central Europe didn't have enough taxable cities or valuable goods, was heavily wooded, would cost a lot of lives and money to conquer, and then would take huge sums of money to to garrison and develop, which explains why the Romans didn't press beyond the Danube.
People beyond the Danube periodically rose up and tried to invade, but the Romans beat them back and went back to the old border.
Why did the calculus change during the marcomannic wars to make Marcus Aurelius try to annex two new huge swaths of territory. Was the land more developed and more easily taxable now?
What was going on?
Although central Europe was more economically developed in the time of Marcus than it had been under Augustus, that was almost certainly not the reason Marcus considered annexing the region.
In the first two centuries CE, the territories north of the Roman provinces of Pannonia and Noricum were gradually drawn into the Rome's economic orbit. The frontiers were among the economically active parts of the entire Empire: the legions were stationed there, paid there, and spent much of their money there. This flow of gold drew in local populations on both sides of the border, who were enriched by supplying the legions with provisions, and often invested their profits in Roman-style estates and villas (see C. R. Whittaker, Frontiers of the Roman Empire. A Social and Economic Study (Baltimore, 1994). By the reign of Marcus, it would have been hard to tell one side of the Danube frontier from the other. So in that sense (despite mountainous terrain and heavy soils), the region which Marcus' new provinces were to cover was indeed more taxable than it had been two centuries before.
Marcus' basic motivations for wanting two new provinces, however, seem to have been defensive. In the first stages of the Marcomannic Wars, the Marcomanni surged over the Danube, defeated a large Roman army, and marched on Italy. They reached and besieged Aquilea before they were turned back - the first time in centuries a barbarian host had penetrated Italy. Although the Historia Augusta and Cassius Dio (our only real sources) do not elaborate on Marcus' motives, it seems most likely that the projected provinces of Marcomannia and Sarmatia would have been established primarily to create a shorter and more defensible frontier - or at least to create a wider buffer between the frontier and Italy.
I talk about the Marcomannic Wars in some detail in my page on the Column of Marcus Aurelius.