@Conker: Basicaly, there's two big material families used for shaped ballistic protection: aramide and polyethylene (PE). Ceramic is another family but being difficult to shape, impact sensitive and heavy, it doesn't really apply to head protection solutions.
Both aramide and PE have their own properties:
-Aramide (such as kevlar or twaron) are, among other things durable and affordable, but are heavy and also UV and moisture sensitive.
-Polyethylene (such as Dyneema) are lightweight and not UV or moisture sensitive. They are however heat sensitive (degrading at about 70/80°C) and also seem to degrade faster over time.
Now there's a ton of different factors modifying ballistic properties: base material assembly (woven vs laminated, type of weave...), layer compression technics (when making helmet shells), and material combinations (different assembly types and/or different base materials mixed together, that's what is usualy called hybrids).
@T1G: is it to say that the ST versions are going away? I'm surprised they would only keep one single solution across the board, even for LEO specific products like LETH
-Are they heavy?
-Then they're expensive, put 'em back.