It's more of an issue with lack of evidence. The only data they have is information provided by the players themselves. While their appearances were reverted to their physical ones, their names were never revealed to each other. Any data logs of:
- Player 09245 "Kuradeel" killed Player 08121 "Bearded dude" at time hh:mm:ss on Day/Month/Year with regular attack.
- Player 00157 "Kirito" killed Player 09245 "Kuradeel" at time hh:mm:ss on Day/Month/Year with ____ skill.
Went with Kayaba and all his server data. Evidence like that would be extremely damning. One player with a grudge against another with a claim that they murdered their friend is less so.
Then again, this is the Japanese justice system we're talking about. Prone to over-reliance on confessions extracted from suspects under duress at police stations and prosecutors with unbelievable 95+% conviction rates (with a certainty of some innocent people sent falsely to prison).