I'm not sure that they concluded it wasn't a crime but it was more the issue of attempting to successfully prosecute in 2021 (after the Dyson report branded Bashir deceitful, dishonest, unreliable and devious) when the victim, Diana, was dead and unable to testify. The original BBC internal inquiry (1996??) cleared Bashir and called him "honest" and "remorseful".
It may also have been difficult from a statute of limitations point of view to prosecute 25 years later.
The BBC did have to pay damages to William and Harry's nanny, Tiggy Legge Bourke (believed to be IRO £200K) for the falsified documents that purported to show that she was having an affair with Charles, which were used to manipulate Diana into giving the interview,
I don't know whether Bashir's medical issues played a part as he was reported to be very unwell with Covid and heart issues when this blew up in 2020-1.