Because she is the most eldest, and therefore most important of the unmarried Bennet daughters. Speaking as an eldest child, I see the logic in this reasoning. As Miss Manners explains, the traditional rule was that if you addressed John and Jane Smith as Mr. and Mrs. Smith, you were indicating that they were “THE Smiths — the heads of the family or its most distinguished members.”
This explanation holds true in Jane Austen’s work. The Bennet family are the principle residents of Longbourn village as Mr. Bennet is the owner of the (inherited) Longbourn estate, thus he and his wife are Mr. and Mrs. Bennet, no first names necessary because in the Meryton community at least, they are THE Bennets. Other examples can be taken from Persuasion and Emma. In Persuasion, Charles Musgrove’s parents are known as Mr. and Mrs. Musgrove (even though we do know that Mr. Musgrove’s given name was also Charles) while Charles and his wife Mary are Mr. and Mrs. Charles Musgrove. In Emma, there is a slightly different variation as Mr. Knightley is referred to as Mr. Knightley by almost everyone while his younger brother is referred to as Mr. John Knightley.
With daughters the rule is that the eldest unmarried daughter is Miss Lastname while the younger unmarried daughters are distinguished by being referred to as Miss Firstname Lastname. Thus the Bennet daughters are Miss Bennet, Miss Elizabeth Bennet, Miss Mary Bennet, Miss Catherine Bennet, and Miss Lydia Bennet; and in Sense and Sensibility, the Dashwood daughters are Miss Dashwood, Miss Marianne Dashwood, and Miss Margaret Dashwood. Each time an eldest unmarried sister marries, the next eldest unmarried sister receives the distinction of beoming Miss Lastname as she is now the most important of the unmarried sisters.
In the case of brothers, the distinction is slightly different as a father’s title or lack thereof can influence who exactly is Mr. Lastname. For example, in the titleless Musgrove family, the senior Mr. Musgrove is referred to as Mr. Musgrove while his eldest son becomes Mr. Charles Musgrove to distinguish him from his father. In Mansfield Park however, Sir Thomas is called such in consequence of his being a baronet, making his eldest son Tom Mr. Bertram while his younger son Edmund is Mr. Edmund Bertram. Marriage does not effect the usage.
So why are younger brothers and sisters sometimes referred to as Mr. or Miss Lastname? Close readings show that this happens when the elder sibling is not present. In Mansfield Park, Mary Crawford is pleased when Tom Bertram goes away because it means that his younger brother Edmund can now be referred to as Mr. Bertram. In Pride and Prejudice, Elizabeth Bennet is only referred to as “Miss Bennet” when her elder sister is not present. Likewise, in Persuasion Anne Elliot sometimes becomes Miss Elliot when her elder sister Elizabeth is not present.