Why does it have so many scripts written on it?

The Rosetta Stone was written in three scripts because there were three different scripts being used in Egypt at that time.

The first script to be written on the stone was hieroglyphics, because it was used for importance or religious documents.

The second was demotic because it was the common script of Egypt at that time.

The third was Greek because it was ordered by the rulers of Egypt at that time to have Greek writing on it seeing they were Greek.