Because it does not buff Luigi to ridiculous levels, but gives him a nice tool to apply close-range shield pressure. Green Missile, with the exception of misfire, moves slowly enough to be easily shielded or avoided at mid to long range. Not to mention how much you telegraph your intentions to get it up to full power, giving your opponent ample time to move out of the danger zone and then approach you from above. If you get stuck in your shield, close enough to Luigi, for long enough to get him charging up the move to max power, you deserve to be punished.
Frankly, I'd like the misfire to travel at Melee speed - it's still too slow in PM for my liking. It was virtually impossible to react even at relatively long range when you didn't expect the move to be attempted.
EDIT: I should also add that when the opponent hangs out at the proper range, the move is super easy to punish with the ridiculous endlag. I'd say it's far easier for Luigi to become trapped in green missile chargeup than for an opponent to be trapped in shield at close range against it.