For what reasons, the campaigns to the east from the middle of the XII century were led not by emperors but by princes?

In the 12th century, the main title of power in Russia was considered to be the prince. In this connection, only princes could lead large military campaigns. In the middle of the 16th century, the title of tsar appeared in the Russian state, as a result, the tsars began to lead the campaigns, and the imperial title arose during the reign of Peter I in 1721. Then the emperors began to declare war and conduct military campaigns.