Well... There are a few regulations I know exactly that contributed to this. One was the roof crush rollover standard. I believe it was passed in 2007 and state

Well... There are a few regulations I know exactly that contributed to this. One was the roof crush rollover standard. I believe it was passed in 2007 and stated that any vehicle must be able to hold up it's entire weight upon it's roof in the even of rolling over and being upside down. The other was the front small overlap test, which hit the vehicle directly in line with one of the front wheels. This shunted the wheel and suspension directly back against the firewall, often completely collapsing the A-pillar. In response A-pillars were thickened and given a much more convex shape at the bottom. Aerodynamics also meant that the windshields are also raked further back. Beyond a specific angle an acute triangle begins to weaken, and the acute angles of many A-pillars and C/D-pillars are well beyond that weakening point, necessitating complex inner structures that one again make them thicker. Since 1996 the IIHS and EuroNCAP have also become more and more stringent with the rear passenger side impact intrusion test, designed to protect children in the event of a side-on collision. One of the unintended consequences of this test was the head restriction height for seats, where cars with higher shoulders (the bottom edge of the greenhouse) in the rear did better in preventing a child's head from escaping the vehicle than cars with even shoulders. This is why only very tall cars seem to have flat shoulders and why small vehicles such as the Mazda3 look like someone canted the entire car forward fifteen degrees.