See the difference?
Frame design, structure and suspenion are the biggest differences. The motor is the same.
The US model didn't get upgraded until 2008. Everything on the US model was a hold over from....1988
The ZZR250 kept up with modern bike styling and found a common medium, which did not look outdated until the late 90s-early 00s.
Don't get me wrong. The US 250 was a great little bike. I had the exact model pictured. But the ZZR would have allowed Kawasaki to basically pull the market out from under other smaller sporty-standards, like the GS500; something the dated styling of the US model didn't allow for.