There's a couple of reasons I can think of off the top of my head.
The green lantern movie tanked, it sank without trace. My wifes reaction to it was "So his power is he can manifest anything he can imagine as long as he concentrates? How can anyone stand against him when he could just imagine velociraptors with machine-guns?".
I doubt you'll see the green lantern on screen for another decade, at least.
Apparently a Green Lanatern movie is in the works! o_O
Also, this is an ensemble movie, we have three known entities (WW, BM & SM) who need to get along & interact etc. They're adding two new characters, who - within the run time of the movie - need to be introduced, we've to get to know them & their powers, and we've to find how they fit into the existing & still evolving dynamic between BM, SM & WW.
I honestly don't think there's room in the movie to introduce more new - and massively powered - characters.
If you compare this to Avengers Assemble, Marvel hardly introduced a new character in that movie, all the avengers & the main villain had appeared on screen before, Clint Barton had the least screen time having had only a small cameo in Thor.
All the super heroes had hours of screen time before being teamed up.
Here we have three fully fleshed characters, with movie time under their belts, but we've to do the narrative work to introduce two more while still telling a coherent story.