RNA and DNA are both nucleic acids, but they have some very different properties. DNA is much more stable than RNA, which means that DNA can be stored for longer periods of time before degrading. This allows for the storage of genetic information in living organisms like you and me.
RNA has a very high rate of hydrolysis compared to DNA. So the shorter your RNA sequence is, the quicker it will degrade into useless single-stranded molecules. This is why a lot of manufacturers use short oligos (molecules that contain two or three nucleotides) as they are easier to synthesize and cheaper to purchase than long oligos (molecules that contain many nucleotides).