I'll try to explain, but I'm not sure that I'll be able to do it clearly. I have two mixed points on this topic. But It's not 100% .

In short, the main reason: Presumably, in order to save Nabu from the point of view of the plot structure of the 4th season, Winx had to use the Black Gift, and not the powers of Believix. And since these are the Gifts of Destiny, it's possible that Nabu was destined to die (season 5 confirmed that Nabu died, and not in a coma/eternal sleep, which is logical otherwise it would be strange that Aisha started dating other guys instead of trying to get her fiance out of a coma/eternal sleep).

Why this is so, I cannot say for sure, but I assume that it may be because, as you know, the power of Believix is weaker than the gifts of destiny. Or because the writers intentionally made Bloom's healing power one of Believix's special spells and nerfed Bloom's ability to heal, making this ability weaker than it was in the second season.

There has always been a question in the fandom about whether Bloom can resurrect a dead person. And I remind you that in the second season, Icy shot icicles at Sky, then there was her dialogue about what subject she should choose to get to the codex, then the guardian pixie and Timmy tried to prevent Trix from taking the codex and at that time Bloom ran up to Sky, who was unconscious, then there was a conflict between Timmy and Tecna related to Trix, but it was only after the Trix left that Flora ran up to Sky to check his pulse and stated that Sky had no pulse and she could not hear the heartbeat. It's impossible to say exactly when Sky's heart stopped, but after Icy's blow, Sky lay unconscious for about 2.5-3 minutes. But the fact is, if there was no pulse and breathing, then Sky was clinically dead and if Bloom had not saved him, then Sky would probably have died completely. It's not known whether the medics of the Red Fountain would have had time to do everything necessary or not. But as it's clear from what happened, Bloom can definitely pull a person out of a state of clinical death.

Now I will explain in more detail about the 4th season:

So, I saw this compilation of spells on YouTube and remembered Belivix's special spells when the Winx immersed people in this colorful 'mental field' with soaring luminous butterflies. Spells such as 'Bright heart', 'Gem of mind', 'Dawn of light' and more.

Then it was even more interesting, because I went to review these moments in the episodes themselves and saw a certain system in the plot structure of season 4. So, in season 4, there are a total of three transformations: Believix is the main one, Sophix and Lovix are additional to Believix, but separate transformations in terms of specialization of powers.

The peculiarity of the transformations of the 4th season is that unlike the first three seasons with the Magic Winx, Charmix and Enchantix, which enhanced the basic elements of the girls such as the power of Dragon Flame, Sun and Moon, Music and so on (adding only universal artifacts like fairy dust ), the transformations of the 4th season don't focus on the basic elements of Winx, but instead adjust their powers to use in certain circumstances. So Believix helps Winx influence people psychologically, Sofix helps influence nature, and Lovix adjusts the girls' powers to spells related to winter.

Another feature of Season 4 transformations is that both Believix, Sophix and Lovix have one special stand-out main spell. Let's take the same Bloom as an example: the main spell of her Believix form is 'Strength of Life', for Sophix — 'Inner Flame' and for Lovix — 'Ice Flame'.

And what confused me was that in Sophix and Lovix, the main spells of girls could be both influencing and attacking/defensive, but the main spells of Belivix for all girls are pure spells of psychological influence. Except for Bloom. It is unclear why, but besides the fact that Bloom calmed people with the 'Strength of Life', this spell is also used as a healing one. The moment that confused me the most was during a trip to Roxy's old farm, when the farmer was injured and Bloom says "Now I'm using my Believix powers" during healing. Not "I'm using my Dragon Flame healing power". Although it's implied that Bloom's healing power was adjusted to Believix and it turned out to be 'Strength of Life'. But Bloom in season 4 never used her healing powers separately, it always went in conjunction with Believix!

Now about the Gifts of Destiny: This gifts have a special plot status, ancient and mysterious. In addition, this Gifts were given to Winx when, after Diana attacks on Gardenia, girls realized that Believix's power was extremely weak against the major fairies of the Earth. Perhaps because Gifts have the status as a higher form of Believix and it used for specific missions, screenwriters did not even think about using Believix spells, considering that Believix is already inherently weak against the background of Gifts and useless during the work of Gifts.

Or the Gifts do have a predictive effect, and Nabu's death is what was destined. But, thinking about it now, and I don't understand why the Gifts of Destiny come exactly bundled with Believix. It looks like a convenient plot addition that doesn't make sense in the bigger picture. WHY does a power that is based on people's faith in magic need additions in the form of powers focused on nature, cold or salvation from death? Yes, formally Sophix is a gift of wisdom and Lovix is a gift of courage, but in fact it's a power of nature and the power of winter.

It becomes even more creepy when you realize that if the Gifts of Destiny have always been bundled with Believix, then absolutely any fairy who received Believix and activated the Gifts of Destiny to help can receive a Black Gift. Forget about Sirenix with his curse imposed by three Ancient Witches. We have Gifts of the power of good here, which, when used, are guaranteed to put someone close to state between life and death.

If ethereal fairies, in addition to Lovix, Sophix and the Black Gift, had other Gifts of Destiny for other situations, then this would be justified, but if the Black Gift is a force constantly going in conjunction with the rest of the Gifts, then, ladies and gentlemen, we have a real curse right here. Of course, it can always be said that the Gifts of Destiny are given only to certain fairies at a certain time predicted by fate, but the potential that someone will die in the process is still there in the conditions of using this 'gift'.