Question: Why is Beetlejuice so intent on marrying Lydia? Why not just find someone else?
Answer: If he marries a human, he can leave limbo and become human again. He needs to get away from his wife, who needs him to complete her blood ritual.
If he just wanted to marry a human, any human, he would not have waited for Lydia. He kept a picture of her on his desk.
Question: In the first movie, Beetlejuice worked with Juno, and later, Otho says that people who commit suicide become civil servants in the afterlife. In this movie, it's revealed that Beetlejuice didn't commit suicide but was murdered by being tricked into drinking poison by the woman he loved. Why would he be working in the afterlife if it was murder and not intentional suicide?
Answer: I thought the same thing on this and also why Lydia's husband was a civil worker, but there's a blink-and-you'll-miss-it part where Beetlejuice is reading a newspaper and there is a small story with a headline titled: 'Workers Wrongly Assigned Suicide at Death', which would explain it.
Answer: Why are most people so intent on marrying someone? He loves Lydia. In the first movie, she was a teenager, and he mostly needed a wedding to get out of his punishment. In the years since, he appears to have developed more respect and appreciation for her. He stopped her wedding to Roy, exposed Roy's fraud, and transformed their almost-wedding into the smaller, "private" ceremony that she really wanted. He doesn't want to "just find someone else" for the same reasons that many people don't.