This is a very good question. The answer must involve Drouet's character. When he meets Carrie on the train he shows his character as a womanizer. Dreiser uses the word "masher" to describe him when he first appears. A masher is a flashy predator who specializes in picking up women on public transportation. He doesn't want to get married; he just wants to play the field. He doesn't want to get tied down. It is significant that his occupation is that of a traveling salesman. They used to call them drummers in Dreiser's time. The fact is that he just doesn't love Carrie enough to want to marry her. Marriage would have meant having children and taking on growing responsibilities and expenses.Carrie meets him again after a long time. I believe he still hasn't gotten married. He is a lightweight; she has outgrown him.
Maybe Carrie doesn't love him either, but their relationship is one of mutual convenience. She would rather be a kept woman than have to work in a factory, and she didn't have many alternatives, especially in those days when it was quite daring for a young girl (she was eighteen) to move from the farm to the big city. It is interesting that when she was first looking for work, someone asked her if she was a typewriter (which was what they called typists). She had no education. She couldn't even type. Her one asset was that she was exceptionally good-looking.