To be fair, there isn't all that much violence—but you can't get away with having the wives and children of the satraps get torn apart and crunched by lions in a PG-13 movie. So, yeah, it needs to be R. You also have the fiery furnace sequence which, true, does not actually kill Shadrach, Meshach, and Abednego. But the two guys throwing them in get horribly burned and die—another potentially R-rated moment. There's no sex or drugs to speak of in Daniel either, though Belshazzar's big final bash was probably pretty crazy, and you'd have to make it look rather lurid if you were going to film it. Basically, it's more the wives and children getting devoured by lions that seals the deal for a hard R.