There are many reasons. Let me highlight the major ones -
1. The Indian Education System - Most people do not do a course because they have interest in it or are passionate about it. Most people do a course because it is ‘coveted’ or popular. The general thought process is - a. Not enough marks to get a decent Science College OR b. No interest in Science. Considering the attitude people have towards Arts, it is a complete no-no for most parents. So what is left? Commerce. Which is the best course to do in Commerce? CA. Hey, lets go for CA! If ever a ‘Why did you choose CA?’ question is asked in the exam, most will not clear!
2. People lose interest - Most people lose interest in CA after joining it. There is quite a bit of degree of mugging involved and you have to slog too much. Subjects like ISCA and law further disillusion the candidates. Eventually, students find alternate avenues.
3. THE ICAI - The ICAI has a standard to maintain. They have to ensure only a limited number of people clear the course, basic demand supply. So 90% of the people who appear for the exams will have to fail. Fed up of multiple attempts, they usually bid adieu to the course.
4. The uncertainty of clearing - The toughest exam of the CA course takes place in the end, unlike the IITs which have the JEE upfront. There is a high chance that even after 4 years of slogging you might never become a CA. Many people are disheartened by this and leave midway doubting their capabilities. They feel they will not be able to cross the final frontier and leave when they still have time to pursue some other course.
5. Paper-checking - We all know how shabby our system of paper-checking and re-checking is. There is a high chance that 2 people, who have written the same answer will get completely different marks. And with the kind of rules in place, it is impossible to challenge the marks you scored even if it is clear that you are right. Hundreds leave due to the fact that ICAI has made this a lot more dependent on luck than it should have been.
I know 100's of people who joined CPT classes with me. Most left by the time Final CA happened - some to pursue BMS, BAF and one even to become a singer!! I think it eventually boils down to understanding what you want in life. There are hundreds of ways to become successful, CA is just one of them.