Why is "Be yourself; everyone else is already taken," considered to be Oscar Wilde's greatest quote?

I agree with experts answers, thanks for the invitation...

It is a beautiful and one of the wonderful which looks at first glance quotations

cuz it's like be yourself it has so much meaning that's why it's one of the greatest quotes 

It is just one of his many fine quotes, that is all

because its true, you can be better or worse

I THINK BECAUSE IT IS IMPOSSIBLE TO IMITATE THE 

OTHERS PERFECTLY , SO IT IS BETTER TO BE YOUR SELF RATHER THAN IMITATE SOME ONE ELSE

I think because it gives all the needed information for self confidence which is needed so much nowadays.

Its concept revolves around self confidence, that one must have in himself. it is most essential ingredients for recipe of life.   

Oscar Wilde is widely quoted and this is just one of of his unforgettable phrases.