Kerr had an easier job than Stevens. Kerr basically took over a project that Mark Jackson built the foundation for. The Warriors were already a playoff caliber team who was expected to improve coming into this season. Of ourselves Kerr has done a fantastic job getting the Warriors to be the best in the West by a decent margin but a lot of the hard work was already done by Jackson (assembling players, coaching them to be playoff-caliber, etc.).
Stevens, on the other hand, took a lottery team in flux, gutted it, threw in young and inexperienced (yet hungry) players, coached them in impressive wins against better opposition, and has them in the playoffs. Nobody saw this coming. Just imagine what he could do with all-stars.