Flying low and fast means that the heavy AA can't hit you, but it leaves you vulnerable to light AA. If you're low and fast enough, then they won't have much warning and your angular velocity will be pretty high, making you harder to hit. At the same time, light AA rounds get to the target faster. That means they are shooting at you, rather than where they think you are going to be. That light AA also has a better rate of fire.

Low and fast was safer in many scenarios, but it wasn't without drawbacks.