They tried. See, there's a continuation to that story. Raker goes back to OA, convinces the Guardians to let him round up an army of Lanterns, and go back to Apokalips en masse.
...Unfortunetly, by the time they get there, Desaad had dissected the Power Ring's functions and realized it's vulnerability to yellow. So when the Lanterns arrive on Apokalips, they're faced with an army of Yellow-clad Parademons.
...After that massacre, I believe the Guardians agreed to leave Apokalips to it's own devices.