It seems that the difference lies in the possible upside. Human clones currently don't seem likely to have much to offer. Conscious AI would have the potential to aid in the solution of a multitude of problems.
Cloning has a huge amount to offer. First things that come to mind are cloning highly intelligent or successful people, or growing brain dead clones for medical experimentation and organ transplants.
We've managed to succesfully halt progress on these technologies and even make them unthinkable, just by having a cultural bias against them. There's no reason we couldn't do the same for AI.
You're definitely right on the second point...especially with a few more decades of advances in transplant technology.
Seems like there is a innate human revulsion towards clones that has aided/led to the formation of this cultural bias. Unsure how easy it would be to replicate those feelings towards AI. Perhaps widespread acknowledgement of the potential dangers would be the first step.
Anecdotally, people seem to have trouble grasping the dangers involved. They think of the iPhone in their pocket and can't possibly see the risk. Obviously it's near impossible to put yourself in the shoes of an entity that could be orders of magnitude more intelligent than the most intelligent human.
We've managed to succesfully halt progress on these technologies and even make them unthinkable, just by having a cultural bias against them. There's no reason we couldn't do the same for AI.