Partly tribalism, I think; it's sort of a badge of Reform-ness to accept Wellhausen. And similarly, lots of Orthodox people only know about Hertz' polemic against it, so rejecting that is part of their Orthodox identity. Studying the issue further and looking at more modern scholarship doesn't really suit anybody's identity. Like, if it's very important to your self-image that your Judaism is rational, modern, scientific etc, you want a kind of simple story about why Torah isn't really "True". And if it's very important to your identity that you believe in Torah min haShamayim, you don't want to consider anything about Torah as a historical document, so you stick with the easily refutable version.