[MattEmily]

Actually Sentinel Knight only restored Adam, Tori and Kira's powers. Bridge didn't lose his powers and Xander's powers were never stated to be lost after his battle with "The Master."

That's true with Jason and Tommy. We can assume with T.J. since his powers were never lost but that doesn't matter in the case of his Legendary Battle appearance since he's not shown as Turbo there anyway.

True we don't know what happened with Ninjor or Dimitria and yes Gosei did give the Megaforce Rangers the powers of the Legendary Rangers.

That's a good thought but not all villains were capable of it. Finster was but Rita herself wwasn't. Zedd was. The Machine Empire was because they were just machines.
Divatox wasn't since hers came from an army and wasn't something she actually created.
Astronema wasn't for the same reason.
Scorpius wasn't and neither was Trakeena since theirs came from an army. Hexuba could because she was capable of magic spells.
The demons couldn't do that on their own except in one case with one of Diabolico's Cards, they had to leave the Shadow World in order to return.
Ransik couldn't do that but there was a mutant that had a Mirror World with previous mutants in it.
The Orgs could do it but only Toxica was capable of it.
Lothor could only do it once the Abyss opened up.
Mesogog wasn't capable of it.
Gruumm couldn't do it.
No one in the Underworld could.
No one among the Overdrive factions could.
I don't recall anyone in Dai-Shi, Venjix or Xandred's Army coming back.
Same with the Armada fleet.