Primarily because the law enabling execution in the State of Arizona (where the hubub occurred) requires a dead certain (pun partially intended) mix of drugs and only those drugs in a given percentage. The drugs prescribed by law are midazolam and hydromorphone. These same two drugs were administered in two previous cases and didn't go well in those events either. The drugs were under evaluation by several States at the time (May, 2014) of the Arizona attempt.
The previous cocktail of drugs that had proven, over a long period of time to be effective and immediate, were pulled from such use by the pharmaceutical manufacturers (who, ironically are European companies) who objected to their product being used is such fashion. Apparently the drugs have other legitimate applications and are still produced.
The case, as can be imagined, is under review. In Woods case (the criminal who murdered his girl friend and her father in cold blood) 15 doses were administered over a period of two hours. Normal expectation is two doses over a few minutes...
The issue of using the Phentobarbital used by the Orgon woman who committted suicide is that it is only available in a few States that permit doctor assisted suicide, Oregon being the most active. The drug, which apparently, has no other uses is only avialable by prescription which weren't possible in Arizona...