Why did Charles V transfer the Austrian lands to his brother Ferdinand I?

Charles V transferred the Austrian lands to his brother, Ferdinand I, primarily because the vast responsibilities he inherited as Holy Roman emperor were too great for one person to manage alone.1

Following his election in 1519, Charles V recognized that his extensive dominions—which included the Burgundian heritage, Spain, and the Austrian lands—required a division of labor to be governed effectively. Consequently, through the Treaty of Brussels in 1522 (following the agreement at Worms in 1521), he formally assigned the Habsburg-Austrian hereditary lands to Ferdinand.12

This decision allowed the house of Habsburg to manage its territories more efficiently, eventually leading to a definitive separation of the dynasty into two branches: the Spanish line under Charles’s son, Philip II, and the imperial line in central Europe under Ferdinand I.34