[MattEmily]

MMPR = Rita had attacked it since it was the closest to the Moon. Lord Zedd had just continued it since it kept on getting in their way and Master Vile followed suit but really all Master Vile wanted was the Zeo Crystal.

Zeo = the Earth was chosen as the Machine Empire's target mainly because of Rita and Zedd's interest in it.

Turbo = Divatox had no interest in attacking the planet until the Rangers had ruined her wedding.

Space = There was no reason other than 4 of the Rangers were from Earth so Astronema just wound up using it to her advantage when she needed one.

Lightspeed = Mariner Bay was built on top of the demons' Palace which is why that was their target.

Time Force and Wild Force = we really didn't have a reason for this. My best guess for Wild Force is because Turtle Cove Lake is presumably where the Animarium came from.

Ninja Storm = Blue Bay Harbor was the target because that was where the Abyss of Evil was at.

Dino Thunder = presumably only because Tommy was there.
SPD = only because of Cruger.
Mystic Force = I don't believe there was a true reason other than they wanted the Light.
Overdrive = There was no reason other than they wanted the jewels and the crown the crowd was originally in Andrew's possession.
Jungle Fury = it was the planet that Dai Shi was trapped in a box on.
RPM = it was likely the last planet that had any defenses for the Venjix Virus.
Samurai = likely because the Nighlok was aware of a new group of Samurai Rangers being put in place.
Megaforce = there was no reason.
Dino Charge = it was when Sledge had lost his asteroids due to Keeper's bomb as a result of Sledge wanting the Energems. He likely figured the gems and Keeper would still be on Earth which is why that was his target.