DNA or deoxyribonucleic acid has C-H bonds between its constituent sugars. These bonds are less reactive and show a high stability in alkaline conditions. The grooves in DNA are also smaller; this reduces the number of locations where enzymes can attach themselves and cause damage to DNA. RNA or ribonucleic acid on the other hand has C-OH bonds between the constituent sugars which increases reactivity and makes them unstable in alkaline conditions. The grooves in RNA are also larger making damage due to enzymes more likely.
To accomplish the task of storing genetic information which determines all the characteristics of an organism a highly stable and non-reactive structure is required. This makes DNA preferable to RNA and is the reason why the body stores genetic information in DNA and RNA plays the role of transfer of information to accomplish the creation of proteins.