Why does John Middleton try to convince Elinor of his lack of money in Jane Austen's Sense and Sensibility?

I was unable to find in Sense and
Sensibility a moment when Sir John Middleton tells Elinor that he does not
have a lot of money; however, perhaps you actually meant John
Willoughby? Towards the end of the book, Willoughby comes to Elinor to
explain his treatment of Marianne, and one of his reasons
is lack of money. Below is an account of his
explanations:

When Willoughby learns from Sir John Middleton that
Marianne was dying of a fever at Cleveland, he rushes from town to try and make both
Marianne and Elinor think "one degree less" badly of him (Ch. 44).

Willoughby confesses that at first when he met Marianne and her
family, he had no other intention than flirting with her and enjoying her company, even
if he saw that she sincerely loved him. His reason was that he already knew himself to
be an extravagant man and knew that his estate at Combe
Magna was not quite enough to pay for his expensive tastes and
debts. Therefore, he also had to wait for his inheritance from Mrs.
Smith; in the meantime, he decided that he must also marry a wealthy
woman. Therefore, Willoughby tries to convince Elinor that his initial
indifference to Marianne, though mean, selfish, and cruel, was due to his
need for money.

However, Willoughby
further explains that he did indeed fall in love with Marianne. He even states that he
had made up his mind to forget about his need for money and propose to her when Mrs.
Smith learned of his affair with Eliza. He never means to justify or excuse his affair
with Eliza, but he does explain that Eliza was just as guilty of seduction as he was
guilty of being a scoundrel. However, due to his affair with Eliza and his refusal to
marry her because of his love for Marianne, Mrs. Smith disinherits him, leaving him in
more need of money than ever. Hence, Willoughby made the resolution to give up the idea
of marrying Marianne, leave for London immediately and court the wealthy Miss Grey
instead.

Ultimately, Willoughby asks for
compassion. Though he made his choices, he wants Elinor and Marianne to
see how torturous it was for him. He wants them to see that he truly did love Marianne
and meant to treat her rightly, but got caught up in his own other foolishness instead.
As Willoughby phrases it, "If you can pity me, Miss Dashwood, pity
my situation as it was then. With my head and heart full of your
sister, I was forced to play the happy lover to another woman!" (Ch. 44).
 

Elinor does indeed feel compassion and pity for Willoughby,
especially because his vanity and extravagance have made him "cold-hearted and selfish"
when in reality he could be "open and honest," with an affectionate nature (Ch.
44).