Why do people buy military surplus or supply?
For one group of people (veterans) it brings back a flood of memories and familiarity. They have used the clothing and equipment and know that it is better than anything produced by civilian firms. We see many veterans who lost items years ago and now want to come back and start using them again. In many cases they buy equipment merely as a keepsake.
For non-veterans, they associate military clothing and equipment as being made to a higher standard of quality. Everything has to be built to withstand a lot of repetitive use and abuse.