how not to see each other
Having two qualified pilots in one aircraft, and one in the other with considerable experience I wonder what really went on up there. I find it almost impossible to believe that they would not have seen each other had they been looking. From the Radar traces it looks like the RV had priority according to visual flight rules, and hence the DA40 should have given way to the RV. Clearly it didn't - either because the pilots of the DA40 were not looking where they were going, or decided that the RV would miss them.