system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo] If the forerunners had at least class 12 combat armour, how could they lose to the flood?

Master Chief's armour (MJOLNIR) is only ranked at class 2. He could easily dispatch the flood. Forerunner civilians were required to wear class 8 at all times and the military had to wear class 12. How come, with technology so advanced, they were wiped out.
gpt
The thing that makes the Flood dangerous is that they gain all knowledge of whoever they infect, which means they can use their technology. You know how you start running into Flood wielding human and Covenant weapons? Well the Flood back then would be wielding Forerunner weapons, which are powerful enough to breach Forerunner armor. Once they had a Gravemind, they would be directed to start putting on captured Forerunner armor as well. 

Because they use your own tech against you, the Flood are dangerous no matter how advanced you are, because they're brought up to your own level. They can also corrupt AI, and a Gravemind is incredibly intelligent and a brilliant commander and strategist, meaning they can usually defeat you once you're on even footing tech-wise.

Also, the restrictions you mentioned only came into effect once the war was in full force. When the Flood first appeared, civilians walked around unarmored. This let the Flood infect the first planets with ease, and gave them the resources they needed to fight the Forerunner military.