Well, the fact is that your traditional rainbow (FFQ, Druid, and Oty along with Quint) dominates t50 because the players are in t50, and it requires a lot of score to do so. Due to the score requirements, people farm FGs, and because you need some source of damage (Best options for rainbow are FFQ/Druid) in a rainbow deck, as well as a lot of control (Oty, Steals, and Pulvy/BE take it again as the best tools available). Since those are such key cards in beating FGs, the majority of t50 players will use them. Can't blame them for farming.
Now, there are some non-rainbow FG decks, but they're not quite as effective and a lot of t50 players don't have them upped.