Because DC and WB hate other superheroes who are not Batman.

No.
They didn't invite Batman to the Justice League anniversary bash, why would they show up in Gotham for something that doesn't concern them? For that matter, they were still partying at the Fortress of Solitude when all of this was going down and Batman earlier busted Superman's computer that would be able to inform him that Gotham needed help.
I would get into why they SHOULD NOT show up for narrative reasons, but the logistical problems of their arrive is enough.
It has been fun fellow message board users.

I knew going in they wouldn't, because the point of the film was Batman accepting the help of the Bat-family instead of going it alone. Just having the Justice League show up isn't personal enough.
"All you have to decide is what to do with the time that is given to you." -Gandalf

One of the first things I told my son. The original teaser hinted at what LOOKED like the Justice League all running togetherthe scene ISN'T in the movie! sad I thought they'd help during the climax, but also kinda glad they made the Bat-villians the "good guys"..kinda went with the Bat Family dance sequence at the end.

Im gonna punch you in the cooter, I swear to God!