Because suspension setup is always a compromise, and the compromises chosen by the manufacturer may not match up to what the owner wants. The SF-325 rode well and was light, but it had softer responses and lower grip than a lot of owners were looking for, especially those that were competing with the car. The same is true today, the OE tire choice may or may not be right for the owner.
The full quote from my chat with David - a full year ago! - is as follows:
“…they weren’t a great tire. We used to call them “Bridgerocks” and tire technology has really moved on in nearly 3 decades.”
It’s worth noting that Mazda and Bridgestone agreed, as they updated the technology of the reissue.
Keith Tanner, Flyin’ Miata