Why do we use a 6mm socket for our axles? First of all, it is the largest size key in a standard pocket multitool. It is very rare to find an 8mm key in multitools, so 6mm is usually the largest standard key size that cyclists carry with them.
Why is 6mm better than 4mm or 5mm? The fundamental difference is the area of contact between the key and the axle, which provides much greater security and ease of use. Such a connection is also much harder to damage (e.g. stripping the Allen socket). Usually, in stock axles, the length of the firmware is from 4 to 6 mm. This combined with a 5 mm key creates a very unreliable situation, and such axles are very easy to damage when trying to unscrew them from the frame or fork. We want to provide the greatest convenience and security to our customers, which is why Hardlite axles have a firmware length of 10 mm and require a 6mm key. This makes the axle pretty much impossible to damage when used properly.