Because currently you have to have a combo going for Gyarados MT to do anything for 1 energy.
You have to have Unown P, Cresselia lv X, or something like that in play so that he can use Carp's first attack. However with Rainbow energy you can just attach and go since Rainbow puts that 1 damage counter on him.
Well obviously because he has 120 HP (good for a stage 1) and can do up to 140 damage with that same 1 energy. He's a nice spread deck counter and a good tech for grass decks. He's just an easy to use tech and rainbow makes it even more so.
For instance you could easily insert a 1-1 or 2-2 line into Scizor, sub out some grass energy for rainbow, and give those fire decks an evil suprise.