Because you can always improve on a design.

Here's the modern helmet maker for the UK fire services: http://www.rosenbauer.co.uk/equipment/helmets/

Yes, since the tasks and material change.

Modern materials are lighter and more stable, thus need less material, which gives more flexibility to move.

Also the tasks changed. These days there are less fires, but more kinds of "technical assistance" and when there is fire water usage is more targeted and restricted.

I don't know - I'm not in charge of the design of fire helmets in the UK am I. I just know they changed and it's a shame it's not the iconic old design we had.

InnoGraft.