Why did military reform take an important place in the transformations of the 1550s?

For the Moscow kingdom in the middle of the 16th century, the issue of protection from an external enemy was in the first place. Crimean Tatars attacked from the south (in 1571 they reached Moscow and ruined it), in the east there was a war with the Kazan Khanate (ended in 1552), in the west they fought from the 1500s against the Swedes, Livonia and the Commonwealth (from 1569).
In the course of the military reform, archers were created instead of squeakers (1550, about 3000), limited localism in wartime, and adopted a code of service.