My thoughts, and some Google research -
* Tee tree oil is not the only oil which reportedly helps the scalp, other oils include neem, lavender and patchouli.
* one website mentioned fungal infections, which can be made worse by going to bed with damp hair.
* it could be product build up or regular use of using chemicals which irritate the scalp. Theres merit in that idea, when i gave up SLS based shampoo, my scalp became less itchy, but then the improvement may also have something to do with the Apple cider vinegar rinse which I was recommended to do after.
* an acidic hair rinse apparently can sooth an itchy scalp.