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

[Robert Vermaat]

Also why the switch from the Imperial- Gallic helmets to the Intercisa  helmets of late antiquity? Was it cheaper to produce? Or were there other considerations behind it?

Also unknown, but there are a few theories.
One is the construction of the last IG-type helmet, which by the early 3rd c. had a very large neck guard. It is thought that due to new (mainly moubted) enemies, a new fighting stance called for a flexible neck guard.
Also, construction in many parts that were riveted together instead of only piece with only the cheekplates attached may have been the answer of the Roman state to the shortage of material due to the massive losses during the Crisis of the third century. New-style fabricae, no longer attached to legions but giant regional complexes would thus have been able to churn out many more helmets than before. These helmets were 9as Always), influenced by a successful enemy - in this case the Persians.