why is Newton's law of universal gravitation called "universal"?

Answer 1

At the time Cavendish measured rhe weigth of the Earth and determined the Gravity constant ,it was called a Universal constant because observations of celestial bodies have shown that the constant applies to all large masses of all celestial bodies including the Earth. Basically all measurements are relative to the earth. It was Newton that first determined how to measured weight as a gravity force and calculated the Gravity constant which is now called Universal.
Neverthe less when dealing with micromasses such as electrons the weight of the electron relative to the nucleous of the atom is related to a gravity constant which has much greater magnetude.Hence the electrostatic gravity constant is not the same in value as newton's Universal gravity constant.

Answer 2

Because it applies to all things. Everything that has mass, has gravity. This was a novel idea at the time, since up to that point many people would have asserted that only the earth had gravity, and everything else was pulled towards it. The notion of universal gravitation shows the earth and all other masses in the universe to have the same basic properties.