Many countries have their own Pre-U examinations. In Canada it is termed as Year 12 or Grade 13. Many students feel that it is very difficult to pass the STPM compared to the A-levels. Some feel that the marking scheme for those who sit for STPM is not fair, so they prefer to take the A-levels. Then there are also those who doubt that STPM will be accepted by foreign universities.
Ignorant students can come up with many more reasons, if they want to.
But STPM is the Malaysian Pre-U qualification and is the only one accepted by local universities. It is also accepted by foreign universities around the world. As the A-levels is a British based examination, it is well known in the Commonwealth countries. It is also accepted in other countries around the world. Students who are intending to study at institutions of higher learning in Britain could sit for the A-levels.