I have to respectfully disagree.
You claim that others are not considering why a Planet Hulk movie would work. I've love to hear an explanation, because I don't see any realistic way that it could.
From a story standpoint, you're looking at a vast departure from anything that has been done thus far. You're taking a character that has, traditionally, not done particularly well in solo films and putting him into a solo film in a setting that not one audience member can relate to. Unless there is going to be a lot of Bruce Banner character study, your audience isn't going to be able to relate to your main character for most of the movie.
That brings me to the logistics angle: For a Planet Hulk movie to work appropriately, you're going to be racking up the bill on the CGI department. You're looking at a HUGE financial risk on Marvel's part, considering that the Hulk has yet to have a solo film perform to the level that they want to see.