Because our 8th amendment prohibits "cruel and unusual punishments" the legal tactic has long been to make every capital punishment seem both "unusual" and "cruel".
Lethal injection seems to escape the charge of "cruel" so the legal battle intensifies to make it more difficult and therefore ever more "unusual".
My impression is the opponents of capital punishment don't even try to hide their true motives in using the legal system to its maximum effect.
Mississippi just passed a law legalizing poison gas, electrocution and firing squad if lethal injections can't be carried out.