لِكَوْنِهِ مُسْتَحِيلًا، أَوْ بَعِيدَ الْوُقُوعِ
Because the desired outcome is impossible or very remote in occurrence.
Mustahīl (impossible) describes an outcome that cannot occur in the relevant circumstances, such as restoring a past period exactly as it was. Baʿīd al-wuqūʿ (remote) describes something possible in itself yet not realistically expected under present conditions.
Tamannī therefore does not require logical impossibility in every case. A highly unlikely but conceivable result also qualifies.