This question has a surprisingly simple answer. As noted earlier RNA first, then changed to DNA. RNA was not an information storage molecule, but was believed to be an enzyme which can self-replicate. DNA, howver, is used to store a long and complex genetic code. Any changes to the code has severe consequences, such as Sickle Cell Anemia. This is the reason why thymine is used not uracil, as uracil can decay into guanine (another nucleotide base) whereas thymine decays into uracil. As uracil is not in the DNA code DNA polymerase (an enzyme) can "see" this fault and correct it. On the other hand, it would not be able to see the problem with a guanine as that is used. So, as DNA storage has to be accurate, thymine has to be used as its decay can be recognised. This is not as much of a problem in RNA as it is not a information storage molecule.
