They may never explain why all the Rangers ar there and why many of them have their powers after having specifically given them up, but it's hardly the first time... They never really explained why it was "only" the red Rangers in "Forever Red" selected for a special mission, for example.
[In my personal canon, Andros tells everyone that the Beetleborg generals had erected a forcefield on the Moon specifically meant to keep all Rangers out, but that Alpha had figured out a [TECH] way to let only red Rangers penetrate it. One call to the morphing grid later, everyone who'd been a red Ranger in the past was back in action much in the same way they did it later for "Once a Ranger".
Oh, and the forcefield also let them have air on the Moon. SO THE ROBOTS COULD WORK WITHOUT SPACESUITS. Yeah, that's it.]