Why did the Four Imams become famous to the exclusion of other scholars?

All praise is due to Allah, and peace and blessings be upon the Messenger of Allah, and upon his family and companions.

The madhhabs of the four imams have been preserved, documented, and widely spread. Their unrestricted rulings have been qualified and their general rulings have been specified, unlike others. It was Allah's wisdom to appoint imams for the people, to whom they could refer for fatwas and knowledge of rulings. Allah also raised those who would codify their madhhabs and elaborate their principles. This was part of Allah's kindness to His believing servants and among His excellent customary practices in preserving this religion. And Allah knows best.