There are American hate sites about William and Kate though - one well known one in particular that many posters here seem to pick up stories from that currently has a new thread for every negative thing said about the Wales’s in Spare to date (much like MN does now). There are hundreds of comments there under each pulling the couple apart (on that site anything positive at all about them is deleted by the moderator, by the way - people claim MN is an echo chamber!). Twitter is also an absolute cesspool.
It’s not even like Meghan and Harry is the only contentious issue on MN that gets people wound up, and sees abusive posts. The other one is far more of a political hot potato though. On twitter, abuse follows any position taken, no matter how innocuous, or so it seems to me.
So this is not a “UK people are deranged bigots” issue. There is a wider issue here about people’s willingness to say abusive things online, and the extreme position taking and partisanship it reflects (the latter being an area where I think the US is by far a more obvious example).
And underneath all of that are the people who just want to have a discussion about an issue that is clearly of interest to many people. The Royal Family is clearly not a niche topic, given their global fame, role in the UK state and interesting history. Many opinions about them and their activities vies are possible. That discussion rather gets lost in the shouting, however.