Simple. Rastakhan was ready to die for his people. He, from the start, was ready to use all of his power to push back the invaders or die.
He was also on borrowed time. He died once before, and was brought back by Rezan. The Loa that was killed by Zul. He only survived by being bound to Bwonsamdi, the Loa of death. Bwonsamdi abandons him during the Alliance fight with him in BoD, because he realizes he doesn’t actually need Rastakhan, because Rastakhan’s foolish deal gave him access to all of his descendants, and shockingly, he has a living descendant in Talanji. (He even shouts this as he leaves Rastakhan.)
However, their pact is still intact, so Rastakhan is still able to call on Bwonsamdi’s power, which is what happens from 60% - 0%.
Rastakhan died because the Loa keeping him alive abandoned him, and he was willing to die for his people, plain and simple.
Jaina, however, was never intending to use all of her power. Her plan from the start, was simply to buy enough time for the other ships to escape. She stored enough power to freeze the remaining Horde and teleport away. She saw the death of Rastakhan as enough and did not want the Zandalari to suffer any more. Don’t believe me? Do the War Campaign on Alliance.
This is the war council, after the battle, while Jaina is still recovering. Everyone else wants to attack and destroy the Zandalari Empire while they’re still recovering. Jaina, the one hurt the most by the raid?
https://i.imgur.com/uzqZHTp.jpg
https://i.imgur.com/DeRxcg0.jpg
She doesn’t believe the Zandalari should suffer more while they’re mourning.
In fact, if you do the Alliance story, the entire point of the siege of Dazar’alor was to kill King Rastakhan, in an attempt to drive a wedge between the Zandalari and the Horde. Jaina was attempting to follow that plan. She did not see a reason for needless killing after the fact.
Jaina is not a bloodthirsty mage that people constantly make her out to be because of the Purge of Dalaran they witnessed on the Horde side. Despite what Horde players think, the Purge of Dalaran they saw is not canon, the developers have stated that many times. She did not murder innocents. She arrested the Sunreavers and allowed the Horde civilians to evacuate peacefully, the only people killed were the ones who resisted arrest and attacked her or the Silver Covenant.
As for why Mekkatorque didn’t die? Obviously they did that because they plan to introduce the Mecha Gnomes as an Allied Race, and he’s the only Gnome Lore figure they have.