I thought that most people thought that Sanditon was Jane Austen recognising the the word was changing and that she needed to reflect it. ie drawing rooms and country parks were not as relevant as the increase in self made men (were there also self made women??) who through money were breaking into a world quite isolated from the hurly burly of life.
And she didn't finish it because she was dying
Who knows what direction her writing would have taken if she had lived into the start of the Victorian era.
She was by upbringing a Georgian, so wonder how much of her novels is in fact that upbringing commenting on a Regency society.
So to continue to bring in money to her family she may well have thought she needed to reflect the changes happening.