The in-game models of each character have many parts that simply can't be removed (the Spy's cigarette is an infamous example).

Less commonly, there ARE removable parts (such as Pyro's head).

Demoman's boots and Scout's shoes/socks are removable parts of the models. No other class has removable feet, meaning that this item is not able to be made for Soldier and most other classes.

For reference, the "feet" cosmetics of other classes actually slip over the already-present default model. The multi-class cowboy boot items are a good example; if you look carefully, you'll notice that the boots are bigger than the original boots.

I just thought I'd state this since there are plenty of comments and even a whole Discussion revolving around making this a multi-class cosmetic.