Publicity. If I had to pick a team, it would be team Sussex, because the roya family has been vile about her from the start, using her as a smokescreen to protect William over his affair and then Andrew over his general disgustingness, but equally I think they were MAD to do the interview, especially Harry. He will really struggle to come back from this, whereas it matters less for her, because she has more freedom (i.e. to leave) and more potential financial resources (she's not as stupid as he is and she has transferrable skills).
I think Meghan has been a victim of Harry's dimness here. The things that she says she was thrown by:
Archie's entitlement (or not) to the title of prince
Curtseying to the queen in private
Booking an appointment with a therapist
These are all basic things that she could VERY easily have been advised on (or googled), unless she had a massively unhelpful partner who was utterly unable to offer her practical support.
Literally any royal or royal aide could have easily told her about curtseying in private. Did Harry just brush her off with "it'll be fine babe - Gran's lovely"?
And I absolutely don't understand the mental health thing, unless she had been warned about privacy and asked to be referred to a trusted therapist or something, and nobody handled it for her, which I suppose is possible.
I can easily see Harry as the type to throw an unhelpful strop about people just having to accept her as the love of his life, instead of saying "Okay, my family is mad, but we can't change that. Here's how to make this go more smoothly."
Or maybe he just dimly assumed it would all come naturally together for her because it did for Kate, notwithstanding the fact that Kate has an amazingly supportive family who have supported her through all the royal bullshit and while I'm sure Doria is lovely in her own environment, Meghan was completely unsupported in a foreign country.
Carole is the sort of Mum who would send you off to your first royal shooting party or whatever FULLY briefed with a spreadsheet of all the people you have to curtsey to and what to call them, and be there with a headset and an ipad under some gorse to keep you on track if you forgot what to do. Meghan needed an ally like that, not a resentful and stroppy child of a partner.