Literally the same as what caused the two 737 MAX crashes and why the law was written in the first place.

There’s no denying that adding these changes would be costly and time consuming, and would complicate 737 MAX operations. So I’d almost feel sorry for Boeing, except for the fact that Boeing lost so much credibility after everything that was uncovered following two 737 MAX crashes. It’s Boeing’s very behavior that’s causing the increased scrutiny around aircraft certification.

Put simply, they want to keep using a loophole that exempts new safety regulations when utilizing an old design. This loophole was specifically closed because of Boeing, and this very same aircraft series, and they want to keep using it. Simply because it ruins their marketing that pilots won't have to be retrained. Peak capitalism right here.