Because it will rub off on other things. At least that's been the case with me and shoes. You might try the brown Pecards, or Meltonian Shoe cream in a dark brown or simlar.
Shoe polish works just fine for small spots and scuffs. It would be impractical over the entire jacket; you just couldn't buff it in, and there are better products out there for that application.
As I answered in another post, if you're referring to liquid shoe polish, it's water soluable and will release if you get caught in a downpour. It can be used as HJ states above, but be sure to seal it with a product such as Pecards dressing to help it be water resistant.
