The most likely reason for any person turning down a full-time job is money.
It is more likely than not that Burger Hut was unable to provide compensation for Debra's services at a rate that a. Debra felt appropriate or b. Debra felt was necessary to make accepting the job worthwhile.
For instance, if Debra lived far away from the Burger Hut than any job would need to pay enough to partially cover the cost of her commute in addition to living and (hopefully) saving money.