The region was already rich before the Bactrian centuries (as mentioned by Justinius), but it got that way for two main reasons: fertile grounds that led to a large population capable of projecting power to its neighbors (Strabo) and its location as a crossroads between India, parts of China, and the Selucids. They were also trading partners with Ptolemaic Egypt. This ability to project power in their region allowed the Bactrians to control trade running through them effectively, and profit mightily.

The difference between then and now is that trade no longer goes through Afghanistan in the way it did in the Ancient world. Bactria and the area to its south were the only real ways of trade reaching East to West, now we have boats (that's a bit of a simplification, but I think accurate).