^From a progression standpoint, Zoro already identified diamond as following steel/iron (and he doesn't even know yet that elements can be cut), so it makes more sense for Zoro to learn how to cut through diamond (effectively meaning he can cut through 99% of anything on the planet (say goodbye to the Pacifistas)) than the elements. It's simple level progression - you can't just skip to level 3 from level 1, you have to first make your way through level 2 (and no, the levels that can be skipped in Super Mario Bros. (etc) are not the levels I am talking about
).