It’s not that Michael Goldenberg and Martin Campbell aren’t talented creatives in their own right, it’s that Warner Bros. hired the wrong folks to oversee a superhero movie. Studio decision-making drowned out any enthusiasm that the original team brought to the project, and the lack of passion is evident in the final cut. The cartoonish CG suits that the Green Lantern Corps members wear feel like Warner Bros. throwing money at the wall and seeing what sticks, which is exactly what the situation was like according to Ryan Reynolds. He told Variety:
“There was just too many people spending too much money and when there was a problem rather than say, ‘Okay, let’s stop spending on special effects and let’s think about character. How do we replace this big spectacle thing — that isn’t working at all — with something that’s character based?’ and that just never — the thinking was never there to do that. And to their credit, it’s a very old school way of looking at things. It’s just ‘Let’s just keep spending our way through this.’ And that was — it didn’t work.”
“Green Lantern” was a casualty of wanton studio spending, an overcompensation for lacking a fully developed vision. Warner Bros. suffered for its own mistakes, losing an estimated $75 million when the film only raked in $220 million at the box office against a $200 million budget (and that doesn’t even include all the promotional tie-ins). Ironically, a superhero whose powers are based on will and courage was the victim of a studio playing it much too safe.
Source : https://www.slashfilm.com/1261116/why-ryan-reynolds-green-lantern-bombed-box-office/