Although these varieties have become less common, that doesn’t prove with certainty that the overall amount spent on treating disease X has not increased. Perhaps these varieties are less common, but people have become more aware of disease X and are more likely to seek treatment for it today.

(B) doesn’t establish anything about the overall amount spent on treating disease X. It leaves open the possibility that the overall amount increased, in which case the conclusion wouldn’t have to be true.

We care about whether the overall amount spent on disease X has increased. But the proportion that disease X spending represents out of all medical expenditures (including things such as heart surgery, cancer, etc.) doesn’t reveal anything about the overall amount spent on disease X.

Even if most disease X spending goes toward nonstandard treatments, and only, let’s say, 40% goes toward standard treatments, we still don’t know whether overall spending on disease X has increased. So (D) doesn’t guarantee that the amount spent on standard treatments for disease X has gone down.

(E) establishes that the overall amount of disease X spending has not increased. If we know that the overall amount has not increased, but the % that went to standard treatments has gone down, we can validly infer that the amount that went to standard treatments has gone down.