Which physical properties of nylon and leather make them good choices for shoelaces?

Flexibility and durability.

Explanation:

The material of leather and nylon is durable which means it can be used for a long period of time and the material is also flexible means it is not hard. The material which is used in shoe laces can be chosen on the basis of its flexibility and its long lasting nature because costumer do not want to invest on them more than one time or on the basis of easily use.

Therefore, due to the long lasting nature means durability and their flexibility nylon and leather is mostly used in shoe laces.

durability and flexibility

look at you shoelaces they are not hard, don't move like a liquid, or have a high density.