Apparently, pre-rendered cutscenes take up a lot of room. I've seen it suggested that the slides used in the epilogue were supposed to be proper footage, but they couldn't fit them within Xbox Live's 2GB DLC limit, so they used artwork instead. No idea if that's true or not, but it sounds believable.
*sigh* I'm really getting sick and tired of consoles being the cause of PC games not living up to their full technical potential. Likewise, ME3's texture quality looks like shit on the PC because they didn't bother to improve the console textures for the PC version this time around, which they DID do for ME2.