I hope I can find the correct words to say this, but I think there are a number of causes behind why we don't have a Wonder Woman movie in the making:
1-Diana is almost perfect: in the beginnings of comics, the characters were "marysue" characters; they always did the right thing, never lost a fight in their stories, and everyone was happy at the end of the issue, and Wonder Woman is no exception. But when Marvel appeared in scene, they made their greater contribution to our little world of comics: imperfect characters. Because of their vulnerabilities, they are easier to identificate with us, mere mortals, and they are easier to write, too. A character who always wins perfectly is very difficult to write without being ridiculous first, and keep the public attention secondly.
This, translated to the film aspect, is the need of a good script. This is no problem since there is a profession called "scriptwriter", although there are sometimes when even I doubt it. So the solution ids hire a good scriptwriter. The plot, after all, is already written.
2-She is a peace emissary: before I make my statement, please don't be frightened if it sounds quite cynic. Being a peace and truth and justice emissary/avatar may sound cool and great, besides, who does not love peace? Those who are dedicated to weaponry industry, the most powerful in the world (sadly). If people realize that waging war is very very wrong, either because of the film or by any other means, it's bad for business.
This is my main motivation to see this movie right made, as an allegate for peace between people, in a superheroic way. Besides, it's always enjoyable seeing the forces of evil defeated.
3-Wonder woman is a warrior: This isn't a problem, except for he who is a male chauvinist. I won't insist in this point.
4-Adequate actress: if Wonder Woman is so perfect, how can we hope to find an actress who can do her justice? the chosen should have, at least, three qualities: good performing, martial skills, and physical strenght and beauty. It is easy to find people with one or two of these fields of experience, but not three at a time.
Last few months, in this site we have proposed many people, we have made a poll... there are forum to discuss this matter specifically. So there are people who can do, if not a great job portaying Wonder Woman, at least a decent one. If all that fails, there always are acting and martial arts classes.
5-A movie is risky inversion: Wonder Woman is well-known in the USA, but not so much in the rest of the world, so making an epic film could suppose the ruin of the studio who makes it, if things go wrong. The whole crew, location, set construction and production costs could easily ascend to 100000000 €, and that's a lot of money, even without financial crisis.
¿Solutions? If the Real Madrid has had a 70 million € (source: http://www.as.com/futbol/articulo/santander-ha-concedido-madrid-credito/dasftb/20090211dasdasftb_1/Tes) then it's half the money the studio needs to do it right. Only with the US box office and merchandising the inversion should be recovered. And the rest of the world box office should provide substantial benefits, so Wonder Woman is profitable after all.
That's all for now. Coming soon in this topic some ideas of how this movie could be done.