So William can sue over photos but Harry cannot say anything about Meghan.
Why didn’t Kate sue? Too helpless?

Well there's a big difference altogether, basically thou William & Kate were the victims of a crime.
It was both
William & Kate - The Duke and Duchess of Cambridge - They took legal action getting an injunction against the
French magazine Closer to to stop it re-printing or distributing the pics of Kate any further.
Which they obviously won as it was
ILLEGAL
This all happened in
FRANCE!
It was
FRENCH police who launched a separate criminal investigation into the charges of breaching
their privacy by the magazine & photographer.
A court in Nanterre, near Paris, opened a separate criminal investigation on Tuesday into charges that Closer and a photographer breached the privacy of Prince William and Catherine by publishing the topless photos...
Their lawyer had argued in court that the photographs were a breach of the French privacy laws and "a shocking breach of their personal intimacy".
The
French Courts Judge awarded the 'damages' to Kate, I guess as it was her pics splashed all over the front pages.
The photographers camera had a high powered telephoto long lens which was set up & used about 1km from the villa
The villa was a private property though to be extremely secluded, This spot was not somewhere someone stumbled on by chance then suddenly noticed Kate & Will (1km) by a quick glance.

(The chateau is owned by Viscount Linley)
Its illegal to even use one of these to focus or aim it's view into private propery.
It's use is banned by the Press Association
This is the lens needed to get the shots
French Privacy Law
Commercial use of a published picture of a person in a public space: Requires consent
Article 9 of French Civil Code states:
“Everyone has the right to respect for his private life"
This is generally considered to include one's right to their own image, even if it is taken in a public space."
WikiCommons guideline - Photographs of identifiable people
Country specific consent requirements
...In a number of countries consent is needed for just taking a photograph of one or more identifiable people, not to mention publishing it and/or using it commercially even if the person is in a public place.
Consent required for action related to a picture of a person in a public place (by country)
...
Country/Territory
France
Take a picture
Yes (with exceptions)
Publish a picture
Yes (with exceptions)
Commercially use a published picture
Yes
WikiCommons
Reuters
The Guardian