Why the switch from Montefortino to Coolus/Iperial helmets happened?

[Corvus]

The rivets would absorb the shock of impact, 

Thats an interesting observation. The modern Crye Precision Airframe helmet is also made of two parts. One of the reasons is to better absorb an impact of an explosion.
https://www.cryeprecision.com/ProductDet...ame-helmet

Maybe fashion played also some role when changing the helmets? 
The Romans always copied equipment from their enemies or neighbors. 

i still dont understand why such a large neck guard on the IG helmets. 
What practical advantage does it provide? 
It only makes the helmet heavier and makes it more difficult to m