How come skin wounds (minor) heal fast in some parts of the body and slow in other parts?

Healing Rates. Many factors. Those areas of the body with the greatest blood flow will heal faster. Those wounds that do not have factors working against them like friction, pressure, shearing, maceration (too much moisture) will also heal better.
Many factors. You also must consider co-morbid diseases (i.e. Diabetes, peripheral vascular disease, tobacco use), current medications (i.e. Steroids, coumadin, (warfarin) etc) and nutritional status of the patient. If their nutritional parameters are abnormal, they will likely have difficulty with healing.