Why does the muḍāf ilayh in شَهْرُ رَمَضَانَ, 'the month of Ramadan,' take a fatḥah instead of a kasrah?

Practical Exercises with the Iḍāfah

• Because رَمَضَانُ is a diptote that takes a fatḥah instead of a kasrah in the genitive case
• Because رَمَضَانُ is the muḍāf and does not have al-
• Because رَمَضَانُ is an indeclinable noun