Why is bacon crispy?

A few things occur to me:

1. Fat content is significantly different between chuck and bacon. Chuck is generally about 15-25% fat (depending on the cut); bacon is often more than 50% fat.
2. The cuts are not equivalent. Chuck is from the shoulder of beef; bacon is usually made from the belly or side. The muscles thus receive very different amounts of work and thus have different textures. (Also, the fat in chuck tends to have sections which are harder and take longer to render.) The equivalent beef cut would be from the plate -- the short plate in particular (between the flank and the brisket) is often used to make "beef bacon." It also has a much higher fat content than chuck.
3. Curing the meat makes chemical alterations that often cause fat to soften and render faster, as well as tenderizing the protein and muscle portions.

In sum, bacon has a lot of fat, is located in a region of the animal with less work (less toughness), and the curing allows the fat to flow out quickly where it can be useful in frying.

What makes bacon crispy is what makes all fried foods crispy -- you dry out the cells by cooking the water out, and the remaining structure firms up. Given the high fat content, a lot of the remaining structure is remnants of the fat sections, rather than muscle tissue (which generally doesn't crisp the same way, even in bacon). Since fat allows cooking at a higher temperature than water, more desirable flavor reactions can also happen (those flavor compounds are also released more in curing). Whereas a more "meaty" piece of meat would go from wet and "boiling in its own juices" to cooked to dry to burnt, the high fat content raises the temperature more quickly but then stabilizes it, which completely alters the reactions that happen in cooking.