They didn't stop evolving. They stopped being good. There's a big fucking difference. Divine Intervention, Diabolus In Musica, and Got Hates Us All are all different from each other and from what they'd previously done. Show No Mercy was NWOBHM on speed, Haunting The Chapel mixed that with sinister, longer tones, Hell Awaits was a diabolical work of raw thrash that really hasn't been duplicated, Reign In Blood was just a pummeling assault. South Of Heaven showed a tremendous sense of the use of tempo to create atmosphere, and Seasons In The Abyss was a further (yet less successful) evolution of that idea as well as further incorporating more ideas.