Why is it discouraged if a person switches from his domain (e.g engineering) to another (e.g hr) ?

There are advantages and disadvantages for this subject:

Advantages: You will aquire new skills and expertise in new field/domain. Some companies are doing what is called "rotation of employees" specially for Fresh graduates.

Disadbantages: You will start from scratch in this new domain.

So, if you have more than Five years of exprience in one domain, stay in it and master what you are doing.