Why does one recite only Al-Fatiha along with the imam in congregational prayer?

Every worshipper knows Al-Fatiha, because it is required in every unit of prayer, and can recite it along correctly. Not everyone knows by heart the other passages the imam chooses; trying to recite them would distract and lead to mistakes, so one listens instead.

The wisdom of this rule is easy to see. No Muslim can offer a prayer without knowing Al-Fatiha. They recite it in every unit of prayer, many times a day. The first verse above calls it the seven oft-repeated verses. So everyone can recite it correctly and without effort behind the imam.

It is different with the other passages of the Holy Qur’an. Not every Muslim is required to know the whole Holy Qur’an by heart. The imam recites what he knows, and it may be a different passage each time. Many in the rows behind him do not know that passage, or do not know it exactly.

If they still tried to recite along, it would not go well. All their attention would go into the attempt to repeat words they do not really know. The prayer would become restless, and mistakes would easily creep in. Instead of devotion there would be hurried repetition that helps no one.

So for these passages one listens in silence, as the second verse above advises. This is not a minor matter but a form of devotion in its own right. Whoever listens attentively takes the words in and follows the prayer with a calm heart.