There is an annotation by David M. Shapard in my copy of the novel that might help explain Sophia Grey's choice of John Willougby.

Fifty thousand pounds was a considerable sum then, much larger than any other female fortune mentioned in Austen novels. Landowning families normally left the bulk of their fortune to the eldest son, or to another male relation if no son existed, to preserve the family estate intact and in the male line. This suggests that Miss Grey comes from a wealthy commercial family, where such practices were less common. Many such families would provide their daughters with enormous dowries to enable them to marry men of higher social status, and thereby raise the entire family socially; someone like Willoughby with a landed estate but serious debts would be an ideal candidate for such an exchange of money for status. In contrast, an aristocratic woman with this fortune would be able to attract a man of even higher rank than Willoughby. The likelihood of Miss Grey's lower background is strengthened by Mrs. Jennings's remembering her aunt, which suggests she knew the aunt when she was living in the commercial section of London with her husband. (Vol. II, Ch. VIII, note 9)