In my experience kids are tattle tales when they have had their power to intervene directly significantly curtailed. I remember a kid asking me to tell my daughter to stop doing some behavior (she was a few years younger than him). I asked him if he had already asked her to stop and he looked at me like I was from mars.

At least where I live, many kids have been taught in many ways that direct intervention is something that will get them in trouble, and tattle-tales are at least partly a result of that.

This is also IMO why girls are more likely to tattle-tale. Girls, on average, get in way more trouble for instigating physical violence than boys, so they have had one method of direct intervention more strongly curtailed.