Hughes 500 - I suspect it has to do with visual clarity and the ability to polish marks and scratches out of the acrylic and possibly strength vs thickness.
I remember the makralon shields well but they were designed for a very different purpose and only had one simple curve instead of many complex ones.
Makrolon is a very old product, it has a slightly 'milky' haze to it.
Optically clear polycarbonates are now common, such as Sustanat.