Why does Daisy choose Tom over Gatsby?

Daisy is an aristocrat who has lived her whole life among the "old money" people.

While Gatsby has become wealthy, he is what we call "new money" people. He has money, but he lacks the elegance and refinement that those who have been raised in money have. He has longed to be part of that world his whole life, and has often admired Daisy because she has the grace and elegance that "old money" breeds. He even remarked that she had "money in her voice."

However, even though he loves Daisy's world, he will never be a part of it, and he loses her for the same reason that he adores her: her high money arrogance.

Daisy may not love Tom as much as Gatsby, but she cannot bear the thought of living in the low class world of "new money". So, she chooses the world she knows (Tom) over the world of new money (Gatsby).