Because it just is. There is no reason per se.
RNA has the base uracil rather than thymine that is present in DNA, so the answer to you question is.. thymine.
thymine
Uracil is not naturally present in DNA. Instead, it is found in RNA, where it replaces the thymine base found in DNA. Thymine is the corresponding base in DNA and is not found in RNA.
One difference between DNA and RNA is that DNA has a nitrogen base pyrimidine thymine that connects with purine adenine. In RNA, thymine is replaced by another pyrimidine called uracil.
The bases present in the DNA of plants are the same present in the DNA of any other organism: cytosine, quanine, adenine and thymine.
Thymine
uracil but that's in rna its thymine in DNA
Uracil
Uracil. Uracil is not present in DNA, but it is present in RNA. DNA's "equivalent" base is thymine, meaning when DNA is transcribed into RNA, the places where thymine would go instead has uracil.
Thymine
There are two purines (adenine and guanine) and two pyrimidines (cytosine and thymine) present in the DNA molecule.
Cytosine is the pyrimidine that bonds to the purine Guanine in both DNA and Rna.