It has nothing to do with you not carrying a balance and everything to do with a lack of history. I got my first credit card when I was 18 and I paid $50 up front to have a credit limit of $200. After 3 months, I got my $50 back and the limit was raised to $500. After 6 months, it was raised to $5500 because I had money in the bank and a good history thus far. From then on out it was easy. I have 5 credit cards now (I'm 22), a car loan, a mortgage, and a small student loan. I monitor my credit monthly and it has only increased with the addition of these other types of credit. You are not required to carry a balance to enhance your credit as I never have and it hasn't suffered because of it. I've paid $0.00 in interest on all 5 cards in 4 years, and my credit rating has only increased.
The number also doesn't mean much sometimes. When I got my mortgage, my average score was 691, and after I paid off my student loan and had the mortgage for 3 months, it went up to 727. Now that I have a car loan on top of the credit cards and mortgage, the average is 745. My 745 is weaker than someone with a much longer history and lower score, however, but the fact remains that carrying a balance does not inhibit you, but lack of history does. They monitor several things once you get a card or two (the max number of cards you should have is 4...I have 5 because we actually need the 5th) like your debt to income ratio, if you have debt on more than 50% of your cards (we only use 2 of them a month and alternate them out to maintain history but keep the 50% rule in check), your max balance, if you have made delinquent payments, and several other things.