why do we call "electron deficient hydrides"?

Group 13 hydrides, mainly Hydrides of Al and B are called electron deficient hydrides.

The elements of these group have 3 valence electrons and thus can only form bonds with 3 hydrogen atoms. After forming 3 bonds, the central atom has only 6 electron around it which is 2 electrons short than what is required for completing the octet.

These hydrides cannot be represented by simple Lewis structure, these have tendency to accept electrons.