Here's why:

* Comets are icy bodies that orbit the Sun. As they approach the Sun, the ice heats up and vaporizes, creating a glowing tail of gas and dust that stretches away from the Sun.

* The tail is what gives comets their distinctive appearance and earns them the nickname "tailed heavenly bodies."

It's important to note that:

* Not all celestial bodies with tails are comets. For example, some asteroids can have dust tails, though these are much fainter and less common than comet tails.

* The term "tailed heavenly body" is not a formal scientific term. It's more of a descriptive phrase used in casual language.

So, while there are other celestial objects with tails, "tailed heavenly body" most often refers to comets due to their prominent and visually stunning tails.