Why does cartilage heal slowly?
Cartilage heals slowly because it has no blood supply.
Cartilage is a specialized type of connective tissue. It can bear mechanical stresses without permanent distortion. But it is a bit different from the other connective tissues. Cartilages are avascular. That means they have no blood supply. They get nourishment from adjacent connective tissues like from perichondrium or synovial fluid.
So, it can easily be imagined that they are always in short of nutrition. Being always in scarcity the cells of cartilage (chondrocytes) exhibit low metabolic activity.
After damage tissues heal by regeneration or scar formation. Chondrocytes are not that able. So, cartilages don't regenerate (except in small children). They heal by scar formation. This is a slow process. Moreover this healing is incomplete.
Diagram : Components of cartilage. There are red line denoting capillary in perichondrium (connective tissue adjacent to cartilage). But no blood vessel in cartilage. The red broken line show the way of nutrients being diffused to cartilage from perichondrium.