This is all to do with The Firm and protecting the Firm.
The Firm is like the Mafia. They control the press and the government and expect total loyalty. Anyone is expendable and anyone on the inside is expected to toe the party lie at any cost.
William is the Firm's man, through and through. He obeys their orders and will never deviate from the party line.
Compare Harry and William's speeches. Harry's expressed anger at the press but was a beautiful loving statement towards his mother. William's statement was designed to gaslight his mother (describing her as "paranoid" TWICE within the space of about two minutes) and was quite literally a call for her own words to be censored. Whatever Bashir did, Diana's own written letter makes clear she was not manipulated into it and would have done the interview with someone else because she was desperate to get her story out.
From day one the Palace and the 'men in grey' tried to gaslight Diana and paint her as mentally unstable to cover up Charles. Whoever said upthread that his affair with Camilla only started 5 years after Harry was born, what rot. Charles and Camilla started seeing each other before he and Diana got engaged, the affair lasted throughout the engagement, and continued into the marriage. The palace gaslighted her to make her doubt her own thoughts and what she was seeing and hearing, then tried their hardest to smear her in the press as mentally unwell to destroy her credibility so no one would believe her when she tried to speak out about how she'd been treated.
It's the exact same tactics the Firm is using on Harry now.
Please don't forget, the Firm has serious money, employs some of the most shark-like PR firms and law firms in the world, and has an inappropriate level of control over the British media. We aren't talking about some nice little family going through private dramas.
For the past decade or more, the Firm has spent millions on the PR campaign to rehabilitate Charles and his relationship with Camilla. They have actively created a narrative of Charles and Camilla as star-crossed lovers cruelly prevented from marrying but finally together, and pushed the idea that all the Diana stuff would have been avoided if Charles had been allowed to marry Camilla in the first place. Which is nonsense. Camilla didn't want to marry Charles (she desperately wanted to marry Andew PB) and Camilla wasn't his only or even his main mistress. The idea that Charles and Camilla circa 1980 were desperately in love and cruelly prevented from marrying is pure PR narrative spin.
The Firm lash out whenever that narrative is threatened because the Firm exists to protect the Monarch and the Heir at all costs. It's why they launched an all-out media attack on the Crown for covering the Diana story and pretended it was because the Crown was fictionalising real life events and turning real people's lives into entertainment, obviously a lie because the Crown had already been doing that for five years without the Palace having any problem with it. It's why they're still trying to gaslight Diana now, and why William has been roped in and ordered to parrot the Firm party line about Diana being paranoid and manipulated.
All sheer PR.