I think the reason the meaning shifted is that the original meaning is not particularly accurate. Although that said, I'm looking into it and it doesn't seem settled that that is the etymylogical root. That root also specifically refers to blood shed in battle. My guess is it was war propaganda.
I think the reason is the shorter version is easier to remember, and people love to attribute genuine wisdom to these kinds of statements. Because the shorter version is easier to remember it is near at hand and got used more.
