'Why are ONC/HNC/HND/Degrees not acceptable to prove competence when applying for a CSCS card?'
Because CSCS cards are intended to prove competence to work on-site and these qualifications do not usually include site-based assessment.
This is why the scheme is mapped to the National Vocational Qualification (NVQ), awarded for competent performance in work-based activities. Competence is proved by completing a range of practical tasks and skills and displaying the appropriate technical knowledge.