Dianabol is a 17-alpha alkylated drug. It's meant for ORAL administration.
Why was it created as an injectible? It's much easier to give a cow or horse an injection over trying to get them to swallow a pill. For us, we're more advanced in intelligence (at least most of us are) and we know how to swallow a pill.
Dianabol is a drug, regardless if it's injected or swallowed, that is active for roughly four hours in the body. It is quickly metabolized since there is no ester attached. Therefore, it's better to take it orally so you can keep your blood levels constant. Also, the injectible version is still a water-based drug. It does not create the oil depot like most testosterone injections.