I wonder what is to blame? If I had to guess, I my instincts tell me that either the focal nature of the drug delivery (small reservoirs do not utilize all the available surface area for drug delivery) or the polymer (it is difficult to have a polymer dissolve and remain "inert" in doing so). I still think the most promising technology advance in the near term would be a durable drug reservoir with a prohealing top-coat.
The obstacle to discovery is the illusion of knowledge.