What stopped the A380 from using trent 800s?

I remember when the A380 (A3XX) was still in it infancy, Airbus were going to use the Trent 800 from the 777 to power the plane. Why did they settle on a completely new engine (Trent 900) when the Trent 800 already had enough if not more thrust to power the superjumbo skyward (90.000lb vs 70,000lb) ?