In my world, it is a mix of Renaissance and 17th century style. For soldiers, they wear helmets as part of their uniform and armor. However, even into their version of the 1700s and 1800s, they continue to wear iron helmets for protection. What conditions would allow them to continue wearing metal helmets into their version of the 1700s and 1800s, like before, rather than changing to hats, like in real history?
For extra notes:
A zombie virus starts to rise. It does not end civilization, but it is difficult to get rid of, and technology can still progress
Gun technology also progresses, and does not stay stagnant
While there are a few breech-loading guns, the majority only use muzzle-loading
When I say "soldiers", that goes for all of them. I am aware of the cavalry still continuously wearing helmets, but I am also talking about foot soldiers as well. In addition, these helmets are not for decoration or ceremony, but mainly for actual protection
For battle tactics, it is a mix of medieval warfare, volley gunfire, and elements of modern warfare (with the exception of some technological limits)
I know that in history, they went back to using helmets, but I am talking about for the 1700s and 1800s. From there, in my world, they continue to use the helmets, not hats