Scaly smelly itchy scalp why?

Seborrhea? sounds like it could be seborrhea or psoriasis in your scalp, assuming you are practicing good hair washing techniques. Try some sebulex shampoo every toher day and consult a dermatologist if no results.
Seborrheic dermatiti. 1. Use Selsun or Sebulex shampoo for several days as needed. 2. Scales can be loosened with application of mineral oil or baby oil followed by gentle brushing or combing of the scalp to loosen scales. 3 consider prescription steroid shampoo if these measures fail.
This site maybe help.
Several things .. This could be related to more severe seborrheic dermatitis, a fungal infection(tinea capitis) of the scalp, or even psoriasis. It's hard to say without being able to evaluated your scalp without seeing it in person. I would encourage you, particularly if it's bothersome, to seek an in-person with your physician. Best of luck.