I would imagine your intended question was: Why does Rainbow cost the same as Classic?
If Rainbow only cost 15 (or even 1 credits, it would make a lot more sense.
For someone like myself, who has 8-10 cards in each clan, but almost every non-CR card from Piranas/Vortex/Berzerk/Nightmare, I have moved from 25 Credits for Elite to 20 for Classic and added 2 more clans to my standard picks. I like selling high cost cards to buy what I want, but I like even more getting new cards randomly (like maybe someday getting a GraksmxxT).
Once I have almost all the cards from the top 6 high value clans, then IF RAINBOW WAS CHEAPER, I would consider moving to random from the whole collection.