Question

why defence is considered as union list

Answer 1

Because both center and state are responsible for defence.....

Answer 2

Defence consists of all those acts and processes that include all the policies that are foreign in nature and also all the steps that are used with reference to the defence and the external and internal security of a country.

This is all related to the security of the states and the nations and therefore it needs to be part of the union list as this particular list contains all matters of national importance.