Because non kit maker has taken permission from government and manufacturer to install there kit on vitara this means once you get CNG install you need to get endorsed into your RC to make it legal which RTO won't do as that kit maker doesn't have certification of that car and also another problem is insurance company won't give you single money if you met with accident as you have done work that require RC endorsed
But Good news is that as demand of CNG Kit has increased in Vitara owner so company is saying after October they will get certification which would make there kit legal in vitara
Its same as changing engine without telling RTO which makes it illegal
any other question