User blog comment:EliteSupreme/problem about powers and transformations/@comment-2056915-20160624100201

Bloom uses healing power in Season 4, either with Believix power or Sophix power, she heals Mike, Mr. Baston, Nabu, Diana and so on, and without fairy dust power. Fairy dust power is just a form of stronger healing power (to what I know so far), and Bloom is not the fairy of healing, she is the fairy of Dragon's Flame, which, by chance, is the creator (I'm talking about the Great Dragon, original owner of the Dragon's Flame) of the magical universe. The reason they have Krystal is probably she is the fairy of healing, so that her healing power is stronger than Bloom's so that they use hers (another way of introducing new characters).

Fairy dust and Mini Winx is still accessible from Enchantix to Believix, as Believix is the higher level of Enchantix.

3 different sets of wings -- Speedix, Zoomix and Tracix -- as well as Gifts of Destiny are aids-on for Believix power. It is understandable why they do not appear again.

Still, as this wiki is created and run for fans by fans, as well as does not in any affiliation with Rainbow themselves, we do not know the reason why they do not (or cannot) use older transformation powers. You could mail directly to Rainbow to ask them, but I doubt if they would ever answer fan mails.