Why is the Fajr prayer not performed in congregation in the mosque like the Tarawih prayer, even though Islamic law commands it?

If the Fajr prayer performed by individuals is the pre-Fajr sunnah (definition) (supererogatory prayer), then this is permissible, because the default for sunnah prayers is to be performed individually. It has not been reported from the Prophet (peace be upon him) nor from the Companions that they used to perform the regular sunnah prayers in congregation.

However, if it is the obligatory Fajr prayer, and people are performing it individually and not in congregation, then they are mistaken due to their negligence regarding congregational prayer, which is an individual obligation (fardh al-ayn) upon able men, and due to their contravention of the Prophet's (peace be upon him) guidance. What is obligatory is to advise them and inform them of the obligation to gather behind an imam who will lead them in prayer.