Different game engines and different priorities. BF3 went with graphics but the core game svcks ass compared to any other BF game before it.
Skyrim is by far the better game overall and compared to most RPGs Skyrim has amazing graphics throughout.
Actually, for the most part, I might say the same for Skyrim.
And no, Skyrim does not have amazing graphics. It's graphics are nothing remarkable. What it does have, though, is amazing art and aesthetics. They're not the same thing.