why, according to catherine, has tom not left daisy to marry myrtle?

In Chapter 2 Tom, Nick, and Myrtle go into the city. While at a party in their apartment, Nick meets Myrtle's sister, Catherine. While gossiping with Nick about Gatsby, Catherine tells Nick about Tom's relationship with Myrtle. She tells Nick that Tom actually wants to leave his wife, but that he cannot because of her religion. According to Catherine she is Catholic.

“She’s a Catholic, and they don’t believe in divorce.”

Nick knows that his cousin isn't really Catholic, but thinks he shouldn't bring it up. Their conversation is interrupted when Nick begins explaining how she is too good for her husband, George, and what is it that attracted her to Tom.