Despite frequent financial failures, the studios have greenlit various female-led action movies, so I don't think that's an insurmountable obstacle, though it's definitely a factor.
I think the single-biggest issue is that there's never been a really coherent conception of who Diana is, creatively, that has been consistently held by writers and audiences. In her original form she was probably the closest thing the Golden Age superheroes had to a creator-owned sort of character -- so much about her is derived from the very unique philosophies and proclivities of William Marston, things that none of the many people who've written since have had much sense of what to do with.
There have been great runs on Wonder Woman since then (George Perez, Greg Rucka), but those runs tend to, like Marston, be highly specific to their creators, and they were never widely adopted by other writers as a mold to follow (indeed, both Perez and Rucka were succeeded by people who basically junked everything they'd done; or, in Rucka's case, editorial dictates caused Rucka to junk it himself to clean the slate for the next guy). There have been attempts to synthesize the best elements of previous runs (Phil Jiminez, Gail Simone), but those didn't really take either.
The Lynda Carter show from the 70s is her only real popcultural solo product outside of the comics, but it's rather akin to the Adam West Batman show in that it offers modern filmmakers no template to work with. It's an icon of camp.
My take is that WW frustrates executives' market calculations because there's no clear sense in their minds of what version of the character the public really wants to see. Hence the constant rejections of attempts at scripting.
