Why are NAND and NOR gates called universal gate?

Answer 1

Short answer because you can implement all fundamental logic operations (AND, OR, NOT) with using NAND and or NOR gates.

NOT a = a NAND a

= a NOR a

a AND b = NOT (a NAND b) = (a NAND b ) NAND (a NAND b)

= NOT(a) NOR NOT(b) = (a NOR a) NOR (b NOR b)

a OR b = NOT(a) NAND NOT(b) = (a NAND a) NAND (b NAND b)

= NOT(a) NOR NOT(b) = (a NOR b) NOR (a NOR b)