Why the US army changed their helmet design?

There have been several changes since Viet Nam. The biggest is in the material used...they're now made of Kevlar. I've only been in 4 1/2 years, and I've had two different types: the PASGT and the ACH. The PASGT was first fielded in 1983, and was replaced by the ACH. The one you have pictured is the ACH.