Also, the Apex board has two 8-pin CPU power connectors and a method to fix fans to the VRM heatsink and M.2 risers (the M.2 fans can also cool the RAM). According to Asus, having fewer DIMM slots (and fewer features) means fewer traces on the board, a cleaner design, and more stable overclocks. Even if you are not into extreme overclocking, stability when running 100% on 10 or 12 cores is a nontrivial problem. The Extreme will probably cost 20%~35% more than the Apex.
Different boards for different users. Be thankful the market is big enough to encourage this kind of variety.