30 years ago, the tire's materials and technology, and even cross sectional shape were different from today's tires. The rubber was stiffer and harder back then, and the tread area didn't extend around the sides so much like many present tires tend to do more of. To maintain contact patch with the road - one had to rely on a certain amount of sidewall flex/deformation, which was precipitated by the lower air pressure in those old tires. The current tire designs use much stickier rubber, more rounded profile and generall shorter sidewalls, so sidewall deformation isn't necessary to maintain contact patch - and actually sidewall deformation can quickly erode the contact patch and tire stability of the modern designs. So, they may be black, round and look like they are made of the same rubber as the tire from 3 decades ago, but there are many differences, and thus, different operating pressures.
Would you want to run MS-DOS 2.1 on your present day 8GB 2.2 GHZ PC like you did back when these machines first came out? Probably not happily!
