Here's why:

* Carbonic acid (H₂CO₃) is unstable: It readily decomposes into carbon dioxide (CO₂) and water (H₂O).

* Ammonium ions (NH₄⁺) are weakly acidic: They would react with the carbonic acid to form ammonium bicarbonate (NH₄HCO₃) and ammonia (NH₃).

Therefore, instead of forming ammonium carbonite, the reaction between ammonia and carbonic acid would yield ammonium bicarbonate and ammonia:

2 NH₃ + H₂CO₃ → NH₄HCO₃ + NH₃