According to vegetius, what led to the changes in roman military armor and training techniques

Originally, the legion had been a heavy infantry fighting force, supported by auxilia from other provinces. The legion was made up of Roman citizens, giving it a lot of prestige when it campaigned. Some idiotic emperor decided to give citizenship to everyone living in the empire, so the Legion lost it's unique appeal as an elite fighting force. As a result, instead of having large legions to patrol the provinces, they were broken into smaller lighter forces, and garrisons were raised instead. Imagine if the US Army was replaced entirely with tanks, but was only fifty thousand strong. Instead of having the army go around keeping places safe, large police forces were raised instead, but if things started going south, the army would roll up in the nick of time. The same principle applies to the legions. Instead of always being at the forefront of the action, which required the heavy armour of the past, the Legions became a mobile force, wearing light armour, using lighter weapons, and traveling faster and further.