I think that's part of it. There's a lot in the poem that isn't modern-audience friendly--the very, very long dialogue, the interpolated stories (that do, in fact, have narrative importance but most readers dismiss as being irrelevant and wasting time), the fact that the poem is often sold as "epic" or a piece of action poetry with grunting Vikings and so when people read it they're inevitably disappointed to find out it's mostly talking with, comparatively, little action. Consequently, when the adaptation is written and marketed, it has to be marketed as action, not as the boring poem people remember being forced to read in high school.

Which for me is profoundly disappointing, because I think a good, interesting adaptation of Beowulf could be done, if writers and producers weren't so desperate to distance the movie from the text it's theoretically based on. It would be hard, because the poem doesn't really seem to tolerate much in the way of interference, but it could be done without being horribly butchered the way Gaiman's done.