Why can you make hard candy with maple syrup, honey, and corn syrup, but not agave nectar?

Okay. So now that I understand the question: the reason you can't use agave by itself is simply that it is mostly fructose, which is more hygroscopic than its friends.

Table sugar, which most recipes call for, is a disaccharide called sucrose. It's composed 50-50 of two monosaccharides called glucose and fructose. When sucrose is heated with water as many candy recipes call for, it begins to break down into its two components through a process called hydrolysis. So instead of having sucrose+water, you have glucose+fructose.

One big concern in candy-making is how to keep your candy solution clear and smooth. The best way to prevent re-crystallization is to throw off the balance with an interfering agent. Glucose (in the form of corn syrup) is commonly called for in candy recipes for that reason. Fructose can be used as well, but the two monosaccharides have different properties, and glucose is generally preferred.

Because once your candy is done, the biggest concern is to keep it from dissolving. Sugar is known to be hygroscopic, and candy will slowly leech humidity from the air and start becoming sticky and soft. It's inevitable.

Which leads into the answer: fructose is far more hygroscopic than glucose. Fructose is so hygroscopic that it can make a hard-crack peanut brittle... Less brittle. If you make a batch of brittle with corn syrup and another with agave, both will work. Both will make hard candy. But the stuff made with corn syrup will maintain a nice crunchable texture, while the agave stuff will start softening and melting right away. It may well get to be chewy like taffy if you try to let it melt in your mouth!

On the same note, you could make stable candy with pure glucose.... But it wouldn't be as sweet, and the texture would tend to be more hard than shattery in my experience. Not bad but... Dull.

And some people think that putting candy in the fridge is the best way to cool it off. If you used agave to make hard-crack candy and then stored it in a high-moisture environment like a fridge, before removing it into a warmer and therefore more humid environment where water will start to condense on its surface, it might seem like it never made candy at all.

High-fructose candy is going to have textural differences no matter what. If that's what you want, then it's no problem. But if you want candy that stays crunchy or melts slowly, then fructose, and consequently agave is not your friend.