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What was the great insight Newton had regarding Earth's gravity that allowed him to develop the universal law of gravitation?

Newton hypothesized that the Earth's gravity might be the force which kept the Moon in its orbit, and that there was a universal attraction among all bodies in space.

Newton hypothesized that the orbits of the planets are actually ellipses, and that gravity causes the planets to move more quickly when they are closest to the Sun, and more slowly when they are furthest from the Sun.

Newton hypothesized that Earth's gravity took the form of long$-$acting waves, which is what allowed it to keep the Moon in a circular orbit.

Newton hypothesized that gravity naturally causes objects to move in a straight line, and therefore an external force must be acting to keep the Moon in its orbit.