In the run-up to the release of Swan Song, most of the British media fell over itself to smear Charles Spencer and failed to do its job.
I don’t think this is even true as a starting point - some are even now offering him a sympathetic platform. The papers reported with glee CS’s claims against KC3, to the point where the palace felt they had to say something. It was reported as very damaging to KC3, with the usual ‘could this bring down the monarchy?’ type questions. The press reported the BP rebuttal, then as the serialisation continued, the book seemed more about KC3 and CS than Diana. People began to suspect his motives, just as it became clear that there were also errors of fact involved. CS could have done really well out of this had he written a better book. He started with many of the public still on side.
I think he lost them because:
a) people find the idea that KC3 was giddy like a lottery winner hard to believe when he looks anything but in all the contemporary footage
b) they find the idea that KC3 behaved so badly hard to believe when contemporary interviews with CS contradict this
c) many still have a fondness for Diana, dislike people exploiting her and would prefer she was left to RIP, especially by the man who said he would never profit from her
d) many have a fondness for one or both of her sons, suspect the book is difficult for him / them and disapprove of CS’s carelessness of their feelings, especially since he swore to protect them in the famous eulogy
e) he wasn’t close to Diana as an adult and wasn’t very nice to her either
f) people find it a bit odd and distasteful to include the irrelevant “don’t forget - Charles has big ears” type stories - what has that to do with his sister’s legacy - again raises suspicions that it’s less about her and more about taking potshots at KC3 (which would be fine if he weren’t pretending it’s all about honouring Diana)
g) the style and content are ripe for lampooning and people find the centring of himself a bit eye rolly generally
h) too many basic errors of fact undermine everything - why should anyone believe anything he says when he can’t be relied upon to factcheck / tell the truth
i) everybody likes to prick a pompous, privileged balloon, makes no odds whether royal or merely aristocratic