Why must I use hardwood? Why can't I use any wood that's available?.
You can use any wood that's available (if you really want to). Taste and aroma are the only reasons for using certain kinds of wood.
The technical reason probably has something to do with chemistry and biology (not my strong point). I think the moisture content has a lot to do with it too. But the differences in wood are still there, no matter what the cause.
It's a little like comparing ragweed and roses. Why can't you cut ragweed and put it into a vase instead of roses? You can (if you really want to). It's just that roses smell so much better.
It's the same with burning woods such as hickory, oak, and mesquite (the hard ones). You don't have to use them. It's just that they make the meat smell and taste so much better than other woods.
There are other good woods for smoking too, such as pecan, cherry, apple and others. Any of these "acceptable" woods work well, resulting in slight variations in the taste and aroma they provide.
I wrote an article a while back about smoking woods. You may want to take a look to help better answer the question "Smoke with What?"
But, history has shown that the recommended woods are the best (based on other peoples experiences). If you want to give it a whirl yourself, though, and experiment with some other woods... please do.
My experience has left a never-ending impression on me. I know now (without question) that I will never forget that some woods will render the meat inedible. I guess we all learn better from experience.