Well personally, I think I just plainly hit the nail on the head with the answer whereas Yamor gave a round the block reply.
Pretty straight forward and clear to me:
Anyway, I think that's your problem, that you've never used a credit card before. One of the biggest things working towards your credit rating is your credit card use. Now, although not using a credit card isn't a minus, you don't get the major plus of using a credit card regularly, and paying it off properly.