There is a number of reasons, such as :

• State-funded Sri Lankan universities are poor in resources. Just few universities generate an additional income to augment state funds, and yet the total costs needed to manage a top-end higher education institute, in keeping with globally completive standards, cannot be matched. 
• Recruitment and ability to retain academic ‘stars’ with excellent teaching and research credentials are next to impossible. 
• Potential for research, necessary laboratory & other infrastructure, essential resources (financial & other) are hardly adequate. High quality research on topical areas in the sciences and humanities is therefore not forthcoming. To attract top, world-class academics and scientists into universities much has to be done.
• Quality Assurance of all aspects of university education is not firmly entrenched within a legal framework, giving rise to many gaps that aren’t filled. Nor is there a robust system for accreditation. Hence, the state universities are not competitive with other globally recognized similar institutions
• University campus environments are mostly not safe and student-friendly for new comers and potential entrants. Recent revelations on various types of inhuman abuse (physical, sexual and emotional) have led to less students applying for university education, and some even leaving after getting placements. 
• Political influence and vigorous political engagement by students, have hampered the academic credibility of these institutions.
• Endowments and philanthropists supporting university academic activity are not common, perhaps due to above reasons.
• Selection of students into universities uses examination criteria only (presently the Z score) and does not take into consideration students’ overall talents, skills & competencies. Perhaps this method doesn’t allow picking the best of the bunch.

The link below shows how the world’s best universities are ranked according to academic reputation, high quality research & other credentials of staff, and performance of students. 

https://www.weforum.org/agenda/2019/07/these-are-the-worlds-best-universities/