Dear businessg101 : Some possible reasons include :
[1] Your written essays were not compelling, did not explain your passion for the school, contained grammatical or spelling errors, or were otherwise deficient.
[2] Your extra curriculars were not as impressive as you might believe - three seasons of sports, music, clubs, leadership positions, impact statements, and volunteerism can all demonstrate school spirit which any University will want to see.
[3] Your rank within your class was lower than you believe - and do not recite the “my school does not rank” because every school ranks by decile at a bare minimum.
[4] Your course load was less than the most difficult offered by your school.
[5] You were not taking a full course load in all five major subjects in both your junior and senior years.
This list could go on for some length, so be careful in doing the comparison strictly based on your numbers.