they are not skin cells

Most well-differentiated cells in human body have poor to non-existent regeneration capabilities. Whilst epidermal cells (top layer of the skin) are exceptions to this rule and have very good cell division rates – this is why superficial wounds heal well and without scarring – the underlying tissues (dermis and subcutis, and the muscle if the wound is deep) have poor ability to repair themselves. Basically the only thing they can do is to generate scar tissue from the few connective tissue cells that still have the ability to divide and produce collagen. These cells do not have the ability to form hair follicles or any other structures that are present in the normal skin, and thus there is no pigment or hair follicles. Also, the blood vessels are usually much fewer, which also makes the scar paler than the surrounding skin.

Over a longer time span, the tissue usually regains some of its original "quality", but it never comes out as good as new, so to say.