re: Why was Princess Diana so revered at the time of her death?

The first and most obvious answer is she was beautiful. Beautiful famous people are easy to project positive characteristics upon and feed into the fantasies of many of being more perfect themselves.

The behavior of Charles and Diana seems highly similar on the face of it but people loved her one hell of a lot more than him. He had the bad grace to be born ugly royal and fall for a more homely women instead of the woman his mother allowed him to marry. Both he and Diana had affairs "after their marriage was over". Both he and Diana have been generous with their time given to charity. The major difference is he is an ugly so and so and she was very beautiful.

It is funny - for reference I think of David Letterman and Johny Carson. Carson was a famous womanizer. He had multiple failed marriages and relationships with any number of beautiful celebrities. Letterman is still on his second marriage but had the balls to both be ugly and had an affair with a cute but unremarkable woman from his show.... and people forgave and loved Carson through it all while they villified Letterman (an ugly man) for having a single known affair. Because frankly, he's ugly, and he ought to be grateful than anyone would have him. Carson was not. He was charming, funny, handsome, and athletic. He deserved to do what he wanted because of this.

That's a major issue that Charles has faced. People feel like he was lucky to be married to a beautiful woman and don't know where that ugly man (who was bullied by his mother) had the balls to fall for a frumpy woman instead of Diana.