In a musical scale, why is an A at 440Hz?

Answer 1

It also sometimes depends on who is playing it.

There are certain orchestras in Europe that tune to A at 445Hz (not sure of the exact frequency) and others at different frequencies, but the majority of musicians around the world
set A at 440Hz because that is what most musicians have tuned to in the past. It is sort of a tradition.

Answer 2

A 440 is the Western frequency, i think that in Japan it 442 or 443. A standard had to be chosen I suppose!