From a fan's standpoint? Because it just wouldn't be the Justice League. Or at least, the Justice League we all know and love. Batman and Superman are an integral part of that. While there might have been times when Batman and Superman weren't present in the League, for the most part, it's always been about them, and to have a Justice League movie without them would just seem incomplete.
From a business perspective? Because Batman and Superman are extremely, EXTREMELY popular characters that have very large selling power and will almost without question pull the general audience into the theater, and help to open them up to and introduce them to all of these other, lesser-known, characters.
Honestly, from my perspective, there's honestly not one good reason why Batman and Superman SHOULDN'T be in a JLA movie. Except for a bunch of whining fanboys who are too obsessed with Batman being "dark and serious" to embrace any other version of the character. Not that there's anything particular wrong with that; Nolan embraces the vision. As did Burton. But that's the thing; you have Batman Begins, Batman, and you will have The Dark Knight. People who like the Justice League don't have anything. Nothing live action. And to say Batman and Superman shouldn't be in it just because YOU don't like that (valid, straight from the comics) interpretation of them is just plain selfish and ignorant.