I have some idea
The reason is when you have low hp then you have blood on face and clothes and when you're going lower then you have more blood and your clothes are'nt so clear
There are a few ways of doing this but they really aren't practical.
The first way is to have alternate "battle damaged" models or skins for each character. This would look good but would take far too much time to make, too much stuff to load and would cause the download file to be too large. Even if each model had only 1 damaged skin/model, it would double the number of them.
Another way would to be have blood decals like it CS:S. However, this doesn't look very good and would make the characters look like they've spilled paint over themselves.
It's a fairly common suggestion and despite not destroying game mechanics, there are other considerations that need to be taken into account.