Why do CUSAT and some private colleges in Kerala rank higher than the more difficult to get in colleges- like CET and GEC-Thrissur ?

It’s true that getting admission to CUSAT is easier than colleges like CET and GEC Thrissur. It has provisions for NRI seats and also conducts many spot admissions. The campus is not very disciplined and the increasing cutoffs have resulted in a huge intake recently. The quality of education has been compromised as a result. I think the reason it’s still highly ranked is that these rankings are flawed and manipulated. It’s better not to refer to these rankings while choosing a college. 

Personally, I will suggest you choose colleges based on the following preference order

NIT Calicut>CET TRIVANDRUM>GEC THRISSUR>Government Engineering College Kottayam(RIT)>CUSAT>Model Engineering College

As you can see, CUSAT comes much later in the preference list so go for it only if it’s the last option.