When/how/why max tire pressures raised from 35 PSI to 44 PSI?

It used to be maximum recommended passenger tire pressures were 35 PSI, now I understand they have jumped quite a bit to 44 PSI. What has changed in tire technology to justify the higher pressures? I still run my tires at 35 PSI , seems like 40 PSI or 44 PSI would be a pretty stiff ride.