I'm wondering if the negative reviews aren't keeping people away. Or other films currently out as alternatives.
I think it all comes down to the fact the marketing never convinced people that Green Lantern was a character WORTH seeing a movie about. The emphasis was all on the CGI and the weird aliens and the green videogame world, and not on why GL is such a cool and interesting character.
At least with Thor, they kinda played up the mystery of who this character was a bit.