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

[Flavivs Aetivs]

As far as I know the Coolus helmets appear about the same time the last Montefortinos date to, as we have a few Montefortinos from the Balkans and Pontic regions that date to the 1st Century AD. The Coolus helmets date between about 10 BC and 60 AD as far as I'm aware.

Coolus helmets though fall under the Weisenau Helmet line, do they not? That is, Coolus, Gallic, and most of the Italic helmets. All belong to roughly the same style but evolved in slightly different directions.