The phosphate backbone has covalent bonds so that it will be strong to keep the DNA bases in their proper order, which is absolutely critical for maintaining the information which they carry. The two strands are hydrogen bonded to allow the cellular machinery easy access to that information for DNA copying and for gene expression.
Work Step by Step
To get this answer, consider the difference in bond strength between covalent (very strong) and hydrogen (weak). Then think about how DNA works, in carrying information in a string of genetic letters which must be kept in order and in how that information is accessed. The bond strength issue then becomes clear: strong to maintain order, weak for reversible access to information.