In this specific context, the author is describing something in the past, namely a bus accident that took place one year earlier. He or she also refers to accidents that happened before last year's tragic bus accident.
When referring to events that took place before another event ("accidents like that") that has been mentioned and that is also in the past ("a tragic bus accident"), one uses the past perfect. So the paragraph should be written as follows:
Last year, a tragic bus accident happened in Kolkata. At least 60 people died in the accident. The bus driver was found responsible for the accident. It is not that accidents like that had not occurred before but the number of deaths had never risen so high.
We know that the author is thinking of other accidents before last year's bus accident because they use the adverb "before".
The situation would be different if we were talking about a more recent accident and we were focusing on its present result:
Sixty people died in a tragic bus accident in Kolkata earlier today. The police assume that the driver is responsible for the accident. A police spokesperson said, "It is not that accidents like this have not occurred before but the death toll has never risen so high."
In the above example, the police spokesperson is talking about an accident that is still being investigated. The investigation and the assumption about the driver's responsibility are two "results" of that accident. To the police spokesperson, the accident is still very much in the "present", hence the use of the present perfect.
