Why did Daisy pick Tom over Gatsby in the end and what does that say about her?

Daisy chose her husband over Gatsby for a number of reasons. She and Tom had a child. Had she left her husband, she likely would have had to leave her daughter behind. Tom would not have allowed her to do otherwise. Secondly, to choose Gatsby would have meant the end of her reputation, and even worse, her banishment from society. Daisy would have been a social pariah, something she would have resented Gatsby for if she'd left. Lastly, a part of Daisy loved Tom, or at least she did in my opinion.