Question

why is candela fundametal unit?
given that candela=lm/sr

Answer 1

Actually, lumen is derived from candela.

"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian."

Answer 2

Some of the SI system's fundamental units are fundamental only in the name and not looking at nature's properties. For example, the electric current is a fundamental unit in the SI system, although it can be derived using the definitely fundamental charge. But time, length and mass cannot be broken down. Temperature could be technically derived from kinetic energies of a system and a mole is "just a number", not something that describes an object's properties.