Having zero debt (and no credit cards) can easily knock your score that low, even if you have a perfect payment record.
The whole credit score thing is an extortion game:
To the consumer: Want a loan? Buy everything on cash back cards.
To vendors: Don't like the fact that we're upping transaction fees? What percentage of your customers use credit cards again?
If they actually were worried about your ability to repay a new debt, the fact that you already have a pile of debt would not increase your score.