It is almost certainty that both sides targeted innocent civilians. The tamils were initially the oppressed and it was a violent civil war lasting more than 20 years.
The lack of interest is because a) Sri Lanka is not a world player; nor are they a threat to global stability. b) The war criminals in Sri Lanka are at the bottom of a very long list of war criminals (e.g. there are plenty of war criminals living it up from the last Kosovo war - or even WW2 Japs).
What can be done about it? Well we could send our special forces in to assassinate these people I guess. But then they wouldn't get a fair trial. Possibly a decent solution if we knew exactly who committed the war crimes and had mountains of proof.
Or we could place sanctions on Sri Lanka unless they handed these people over. But then cue all the bitching about acting like world police. Not to mention the many innocent deaths sanctions would bring.
Truth is doing anything now would cause more problems than it would solve.