DNA, being a double helix, is more stable than RNA. However, it's hypothesized that the earliest living cells actually used RNA as their genetic material. While RNA is only a single strand of codons, it provides ESSENTIAL functions within the cell, so evolutionarily speaking it doesn't really make a lot of sense to get rid of something that works so well, but it makes sense to improve on something that works so well. Hope that helps.
Also since DNA is the universal genetic material and that almost every organism uses DNA, the five (ATCG in DNA, AUCG in RNA) nucleotides haven't changed much in the past hundreds of millions of years
