Curious as to why can't the body successfully heal an achilles tendon injury or degeneration?

The body can: If yours isn't, then you need to seek medical attention to find out why and what can be done.

Vascularity: There is not a lot of circulation to the heel cord where it attaches into the back of the heel. Relatively less circulation means less delivery of nutrients. Good medical and surgical care of his condition is imperative. See a podiatrist or orthopedist for care of this.