I think Sansa as a character was harder for people to like. She was more complex than
Arya, who was a pretty one-dimensional badass. I have heard a lot more criticism of how Sansa developed as a character. I know plenty of people who hate Sansa and find her an insufferable character, but she turned out to be one of my favourites, while
Arya bored me a bit.
In the same way, Sansa is harder to enjoy as a name than
Arya. It's a bit weirder.
Aria was already kind of trendy, so
Arya didn't have to work very hard to get attention; it just put a little twist on something people already liked.
I do wonder, if the names on the characters had been swapped, if Sansa would have become popular? Or if Arya/Aria would have risen as much as it did?