Because they could spread misinformation or selectively disclose some information while withholding information that could harm them.
It makes a ton of sense to not let the suspected parties join the investigation.
Well, you could make the same argument in court. Why allow defence to present their side of story? What if they lie and spread misinformation. Better not listen to them and just make a judgement without them.
By not allowing Russia to present their story because “what if” you are preemptively saying that they are guilty.
By not allowing Russia to present their story because “what if” you are preemptively saying that they are guilty.
Why was Ukraine allowed to participate?