Transsylvania was a Principality in the Kingdom of Hungary, which enjoyed a semi-autonomous status in the Middle Ages and when Hungary was cut into 3 pieces in the 1500's became a place nominally an Ottoman vassal, practically balancing between Ottomans and Austria.

Hungarians and Romanians did about nothing with the natural resources due to the lack of know-how. In the Middle Ages German settlers were invited to open mining towns. They were generally called "Saxons" but had nothing to do with Anglo-Saxon, nor they had much to do with Sachsen in Germany. It is not really know where did they come from. One theory says some came from Sachsen and the privileges they got were also extended to others so their social status was "Sachsen", others say the first wave of settlers got killed by the Mongols and the second wave were really Sachsen. It is not know where exactly in Germany did most of them come from but no reason to assume it was Austria. According to some linguists, their dialect resembles that of Trier, to others, that of Luxembourg. It is due to them that Transsylvania has a German name, Siebenbürgen etc. basically they created the German aspects of the culture there.

In the 1500, much of Hungary came under Ottoman rule. Northern and Western parts under Habsburg, Austrian rule and Transsylvania as above. Eventually Habsburgs kicked the Ottomans out and also managed to gain power in Transsylvania. It is not documented that they would have invested in building up the place or moved there in larger numbers. Transsylvania was known as a rebellious place, unsafe. And no naval route, and land travel or goods delivery from Vienna to say Gyulafehérvár wasn't too convenient before railroads became a thing. The "Saxons" (Transsylvanian Germans) went on with the mining and presumably improved the technology as the time went by. Austria mostly invested in Czech mining and industry.

Source: Hungarian high school textbooks