Many different reasons.
For me it all changed when they started promising people jobs as CFI's before they had even flown an airplane. Students stopped studying as hard because they knew the job was in the bag. It was kind of the one safety net that was in place, that the bottom half would not be back to instruct.
Bad students became bad instructors and like making a copy of the copy things were getting worse. To top it all off they introduced the "private-insturment" deal, which was another total catastrophe. I was leaving around that time but saw quite a few people get bounced out and stuck with a huge bill (the refund process is nuts) and literally nothing but a student pilot certificate to show for it. Those that did finish were left with (in my opinion) a very low level of skill for the certificates they actually held, and were right back creating the next generation.......