What enables azacitidine to incorporate into both DNA and RNA?
If you look at the very beginning of the Wikipedia page, you'll see that azacitidine is an analog of cytidine, the C letter of both DNA and RNA fame. Decitabine is a deoxy derivative, and hence would only be incorporated into deoxyribonucleic acid, or DNA.

Reading further, by structure azacitidine is ribonucleoside derivative, and is preferentially incorporated into RNA. However, it can be metabolized to 5-aza-2′-deoxycytidine-triphosphate and so be incorporated into DNA.