I do not have enough information to say what exactly is the problem with HP's memristor. The axiom of technology development in NVM is:

"The technology of the future will always be the technology of the future."

Just like cold fusion has been 5 years out for a long time. I know that is cynical, but the truth is flash keeps scaling and the new technologies are not cost effective compared with silicon flash.

evolution > revolution

The second link did a good job of summarizing the technologies that existing companies are pursuing. Most of these companies would like to have a two terminal element (resistor) over a three terminal element (transistor) if they could. It requires less space. When one of them finds a way to make it cheaper than flash you will see it on the market.