Maybe it's because enough people find 3 inches to be plenty for most Edc needs? If you find that you need a 5 incher, that's cool too, but I certainly Have no practical use for one. As a result, I fail to see the point in adding extra weight, bulk, and cumbersome length to the end of my knife. I've tried carrying bigger knives, but I find that it only does two things......scare more people and make small tasks a harder. With length also usually comes greater thicknesses, meaning that your cutting geometry typically starts to suffer as you get into larger and larger blades. I feel much better carrying a nice thin 3inch cutter than a big old 5 inch tactical blade that WEDGES stuff apart
For me 2.7-3.5 is right about perfect for my needs. Anything else is unnecessary, and just makes me feel like I'm doing it "because I can".