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

I wonder if the performance difference in tire pressures diminish as pressure rises? i know its a big difference seeing 5 PSI compared to 10 PSI, but the performance of a tire probably does not change that much when we are comparing 30 PSI to 40 PSI. Once the tire gets more or less "filled" it won't expand that much more with more pressure (or get much stiffer), I don't think anyway.