This is a very complex question with tremendous social and political ramifications. I can only give my opinion on this, and other posters from the academic community, such as Gladescat, may have different perspectives. My best answers are these:
(1) Lack of preparedness among incoming students. (a) Teaching at the grammar school and HS levels is not what it used to be. Much of this decline in preparatory education is attributable to the fact that college-educated women in the 30s,40s,50s, and even the 60s had few opportunities outside of nursing, teaching, and secretarial work. Very bright, dedicated, and competent young women became school teachers. Those women today are doctors, lawyers, and business people. Today's teachers, quite frankly, are not of the same caliber. The loss of professional pride and community respect (unionization), loss of control over curriculum and classroom discipline, and the need to teach to standardized tests have all further eroded the attractiveness of teaching. (b) Less focus on thinking, reasoning, and writing and more emphasis on learning facts and memorization in HS. This is partly due to the lower competencies of teachers and partly due to teaching to tests and standards. It is also combined with a new emphasis on judgment and emotion over sophisticated thinking. Today's frosh can tell you something is good or bad or that they like it or do not like it, but are stumped to present good arguments in support of their positions. (c) SAT scores have declined nearly every year since 1975. The tests have been recalibrated (at least twice, maybe 3 times) so that the decline in the scores is hidden. One reason for the decline is that more students are taking the tests. College is now seen as offering a necessary degree and is generally accessible to all. (2) Students today see college primarily as a trade school, offering training necessary for a job. This was never the intent of college. For example, Kinsey started his groundbreaking study on sexual behavior by asking his students about their sex lives. Students and faculty composed two sets of adults sharing academic inquiries. Few college students have such a mindset today. (3) Maturity - emotional and intellectual. Helicopter parenting and other social trends have tended to infantilize young adults aged 18-25. (4) Thirty and forty years ago, profs could assign a book a week, offer no discussion of the books in class, and have questions about the books on exams. That does not happen today. Students simply refuse to take classes with "heavy" reading requirements (3 books per semester is about the limit today) and expect to discuss all of the books in class. They generally cannot discern thesis statements or themes without aid from professors. Sad but true. (5) Lectures have largely given way to discussions. Students "need" to participate and have little ability to concentrate for an hour-long lecture. The stories about lower attention spans are all true. Sadder yet. (6) Grade inflation and a\institutional desires for tuition basically ensure even the worst students of graduating with a B- average. (7) Because the majority of college students are incapable of writing essays, multiple choice exams are much more common than essay exams and papers. Many professors from all over the country have told me that they have simply given up trying to teach students how to write. The students resent a focus on writing and administrators fail to support the professors when enrollments in "hard" classes decline.(8) Political correctness has hindered classroom discussions, shaped curricula, and rendered students more sensitive to hurt feelings than interested in debating tough subjects. Seriously, I could go on, but I think you see my point.