One reason would be that not all people are really sure they want a permanent, good for the rest of your life tattoo. Many people want to experiment with the idea of having a tattoo before committing to it for the rest of their lives. Since the henna does not penetrate the outer layer of the skin, it will fade over time and finally disappear.
While the henna tends to produce only dark colors, it can be made into several shades, all of which are dark. On the outside limits, the henna impression could last about a month before fading. But in reality, it will probably last no more than 7 to 10 days depending on how good the artist was who did the work.