Your question is based on a wrong assumption: not only lemon juice can be used to clean copper. Any weak acid, e.g. vinegar or orange juice, would do the trick. The chemistry behind it is simple. An oxide layer forms on the surface of the copper item, such as a penny, when it gets in contact with air, especially moist air, for a long time. If you expose a metal oxide to an acid, such as the citric acid in lemon juice, the oxide layer dissolves. This reveals a shining copper layer that was hidden underneath.

Lemon juice and vinegar are excellent environmentally friendly cleaning reagents for all kinds of purposes. You can safely clean your whole kitchen using these ingredients.

You can use other liquids to clean copper, but the citric acid in the lemon juice works very well at cleaning metals. Many acids are good at cleaning, but since lemons aren't too dangerous (other than the stinging the acid can cause if it gets in your eye or a cut), it can be a good idea to use lemon juice to be safe and keep the environment clean.