Because the RNA molecules that are in most organisms can’t replicate themselves. Further, the RNA molecule that is in most organisms can’t transcribe its information on the DNA.
The DNA molecule readily replicates itself under the right conditions. The reason that DNA molecules replicate themselves much easier than RNA molecules is because DNA has a double helix structure.
The double helix can unzip down the middle, creating two molecules with complementary information.The RNA is generally a single string of base pairs. RNA isn’t double anything. So an RNA molecule usually can’t divide into two molecules with complementary informaiton.
That being said:
RNA molecules in some virus species can replicate itself. This RNA can transcribe information onto the DNA of the host organism.
RNA molecules with a certain structure under the right conditions can replicate themselves. However, replication in RNA is not as reliable as replication of DNA. That is one reason that RNA viruses mutate at a much faster rate than most DNA viruses.
