Because Philosopher is a British / English term for saying Sorcerer, as where Sorcerer is a American term for Philosopher, the Publishing company felt that it was a good idea to change the title so the American readers would not be confused since JK rowling is English.
It was initially released in the UK as the Harry Potter and the Philosophers Stone as that was its original title. It was then released in the US with the title Harry Potter and the Sorcerers Stone because Scholastic, the U.S. publisher, "thought that a child would not want to read a book with the word 'philosopher' in the title."
One was used for the original UK release (The Philosopher's stone) and the other was used for the US release (Sorcerers). I believe the reason was that the US audiences wouldn't "get" the Philosopher part.
