Here's why:

* Bismuth's Common Oxidation States: Bismuth (Bi) commonly exists in +3 oxidation state. While it can theoretically exist in the +5 oxidation state, it's extremely unstable and rare.

* Stability: Bismuth(V) compounds are highly reactive and tend to decompose readily, making it difficult to isolate a stable Bismuth V oxide.

The most common and stable bismuth oxide is Bismuth(III) oxide, with the chemical formula Bi₂O₃.