Why isn't there a tropic of Aquarius?

Answer 1

There was no room for it.

The tropics of Capricorn and Cancer mark the most southerly and northerly (respectively) lattitudes at which the sun appears directly overhead at noon. At the time they were named, the sun was in the direction of the two named constellations. It was not in the constellation of Aquarius or any of the other ten signs of the Zodiac, hence there are no tropics named after them.

Answer 2

The tropics are defined by the location of the sun at its zenith, creating the Tropic of Cancer and the Tropic of Capricorn. The concept of a Tropic of Aquarius is not based on the sun's position as it is for the Tropic of Cancer and Capricorn. Astrologically, the Age of Aquarius is related to the precession of the equinoxes and not the position of the sun in the sky.