The JLA didn't step in because Batman told them to.
Batman: This is MY CITY. I'll fix it. JLA: Really, Bruce? You'll use your super-strength to dig people out of the rubble; your super-speed to deliver food, water, and medical supplies; your invulnerability to go into hazardous situations, such as fires, gas leaks, and toxic spills, and clean them up; your cosmic energies to repair key infrastructure; and your telepathy to coordinate everything? Batman: ...I'm Batman.
Even then, the JLA doesn't have to rebuild everything. They could just render basic humanitarian aid. But they didn't, because the US Government declared that Gotham City was no longer part of the USA, enforced with a military blockade and orders to shoot at anyone going in (and possibly out too), and because Batman said so.