Question

In ray diagrams why we always place objects above the principal axis

Answer 1

There is no problem if we take the object below the principal axis but it will be difficult to find where the image forms and the nature of the image etc.
Simply it a common method.
Only if we make some standards for explaining such things,all can understand the nature of object,where it is formed,what type of lens or mirror is used etc.
So object is taking always above the principal axis by all doing this kind of lens or mirror problem
And it is a standard thing that all are following