Because pointing out that Meghan and Kate received a lot of the same criticism doesn't fit the narrative that the lovely perfect white woman has "never put a foot wrong" and the mean evil black woman deserves all the hate she has received.
It's just how royalty works, there always has to be a scapegoat so the media can get their money's worth (royals getting along, quietly doing charity work and not causing any scandal doesn't sell papers) and that scapegoat is almost always a woman, nearly always a married-in and not a blood royal, and never an heir because they're well-protected.
They did it to Diana, they did it to Fergie, they did it to Kate, and they did it to Meghan. Meghan had it worse because of the additional aspect of racism and the very real danger of stoking white supremacist groups (the fact multiple people who are members of white supremacy/neo nazi terrorist groups have been convicted and sent to prison for either plotting to kill Meghan's family or calling for them to be killed, proves how dangerous it is).
It also exposes the rank hypocrisy of the tabloid press, who so frequently praised Kate for doing the same things Meghan was demonised for. Eg Kate eats avocados, she's health-conscious. Meghan eats avocados, she's linked to war and drought. Kate wears black, she's stylish. Meghan wears black, she's breaking a non-existent protocol. Kate cradles her bump, she's a loving protective mother. Meghan cradles her bump, she's attention-whoring. If Meghan wasn't around, Kate would still be trashed for those things too. Look how viciously the press trashed Kate for wearing pink to a memorial, or for forgetting to put weights in her hems and accidentally flashing her bare arse a bunch of times. Yet now there's an Orwellian "we have always been at war with Eastasia" thing in the relentless repetition of "Kate has never put a foot wrong." (Seriously why does her PR team always use the exact same phrasing? Such a giveaway that an article comes from her team.)
Or the hypocrisy over housing and the press being selective in when they're outraged that so many royals and royal-adjacent people get free houses and spent zillions of taxpayer money doing them up. Will and Kate spent hundreds of thousands of pounds refurbishing their mansion at Kensington Palace on the grounds it would be their forever home, then decamped to Anmer, spent even more vast sums (eg having a brand new kitchen ripped out then completely re-done twice because Kate couldn't decide on a colour, had the tennis court moved a few feet so it'd have a nicer view), again on the grounds it would be their forever home, now they're moving yet again and maintain at least three homes, use helicopter frequently to travel between them. Then everyone pretended that had never happened while Harry and Meghan were being trashed for using their own money on essential maintenance for Frogmore Cottage, which should never been allowed to fall into such a bad state of repair in the first place considering the monarch has a legal obligation to and receives public money for maintain listed buildings in good conditions.
And now Harry and Meghan are gone, the press are sniffing around for new victims and Kate's being trashed for the same things she was praised for while Meghan was around.
There was a big article in Vanity Fair just a few days ago trashing Will and Kate for being so profligate in spending "insane" amounts of money refurbishing all their different houses, the article also claimed there are currently more than 100 "grace and favour" royal residences, down from more than 200 during the Queen's reign. We've seen a rise in articles trashing or making snide insinuations about Kate. The Middletons were clearly thrown under a bus over their company going bust and their failure to pay debts to small businesses, whereas a decade ago their debts would have been paid and the media controlled. There's an increase in articles trashing Charles and Camilla. It's obvious that Charles and Camilla's camp and Will and Kate's camp are briefing against each other.
It won't stop. And it's only a matter of time before they start in on Charlotte and whoever George and Louis marry, if they do marry.