For some cars (my Subaru SVX), the dash was curved in such a way that the sun always reflected on the inside of the windshield. During the day, it was quite annoying. Also in some parts of the country, Phoenix in particular, the sun tends to beat down on the dash and destroy it. Personally, I think the later reason is a throw back to the old days and dash material is very different now and quite durable.
Those are the two reasons I know.
