Why? Maria follows the forums and is perfectly aware there's tons of theories on Cyndane. That's why she hesitated to answer, though she finds the Cabriana theory so stupid (it's shot down in LOC itself - Cabriana's and Cyndane's descriptions were not a match) that she saw no harm in shooting down that detail).

They're holding back things like this because they're fully aware of how much the WOT community thrives on theories, and has been for years. For many of the WOT fans, it's half the fun. The more they give final answers in Q&A, the more they reduce the pool of theory topics or narrow down the possible answers, and a lot of the fanbase don't like that.

She won't give a final answer about Cyndane because she knows she's a huge topic of speculation, not because there's a reason to speculate about the origin of her body (mind you, by casually answering as she did, she indirectly confirms she's Lanfear, another question they refuse to answer when asked directly... because some people have this crazy notion that she's not Lanfear). By all evidence (ie: her northern looks), Cyndane's body is a random one taken from the Borderlands, like Halima's, like Dashiva's, and from the description too, Moridin's. Shaidar Haran said so straight out in LOC - that bodies for transmigration were the best suitable bodies taken from raids in the Borderlands. It's fans and not RJ who made a big mystery out of this: he gavce the answer right the moment the transmigrated Forsaken were introduced! We won't get an answer, because people have made a big deal out of it and turned this into a favourite topic of speculation, that's all.

As he explained in a Q&A, Brandon/Maria have a slightly different view about this than RJ did. Brandon more than RJ thinks there's no harm in shooting down the more far-fetched theories when he doesn't need to reveal too much to do so. He prefers to correct things when he sees people are grossly misinterpreting something. He doesn't enjoy seeing people having it all wrong particularly (and for his own books, he sets them straight). RJ did that too, but not a much lesser extent. RJ was in a different position too. Unlike Brandon who knows pretty much what he will and won't answer in the books - because on most issues it's not his choice and he only has a limited number of items on RJ's list marked "maybe" and a whole lot of 'leave unresolved" and "to resolve" - RJ liked to keep his options open. The details of the books were never set in stone, he couldn't be sure he wouldn't have an idea down the line to return to this or that detail left unexplained earlier. Fat lot of good it would have done them if he had already given away those details in Q&A. He started giving more answers about stuff he wouldn't use when he reached the final stretch, around KOD.

Brandon/Maria are more talkative than RJ (too much to many people's tastes, they've shot down a few popular theories), but also still refrain from shooting down theories that make sense, wrong or right. They wait for the series to end and people to see what gets answered and not.

They have an encyclopedia in the works that will answer a lot of the backstory and worldbuilding questions (OP etc.) too.