The reason movies with graphic bloodshed and gore get an R is because of how gross and disgusting it is. It can be a cause of repulsion and can cause some to lose their appetite. That being said, it's all in the context, however the R rating comes from that. The blood that is.
Violence, such as murders and war violence, are another thing, since a gunshot will not necessarily cause someone to bleed externally. But as far as gore and blood itself, it's because of that.
It's disturbing but also disgusting as well.