Because that 860 page report was written by his team to exonerate themselves and their allies and blame their rivals and whistleblowers because the party were being investigated by the Equality and Human Rights Commission, an independent investigation still in progress
Irrespective of the merits of the claims made in the report - itself the subject of further independent investigation and lawsuits filed by people criticised in it - it's not something which would have been written if things were going well.
All of the things that you claim are evidence of a problem are only evidence of a public perception of a problem. In a world where they are guilty, and a world where they are innocent and it's all a smear campaign, the investigating bodies respond the same.
Irrespective of the merits of the claims made in the report - itself the subject of further independent investigation and lawsuits filed by people criticised in it - it's not something which would have been written if things were going well.