One reason for the suspected overfishing is that shark fishing often takes place in international waters where no one has authority to declare sharks off limits. Another reason is that there are areas of the world where no one knows how many sharks of which species are being caught but there is great suspicion that the numbers are high. WWF says that 125 countries are in the business of catching and eating sharks to the tune of 30 to 70 million sharks a year. What is interesting, however, is that according to WWF, most sharks aren't caught on purpose. Elsewhere, sharks die of oxygen starvation when nets trap them and prevent them from moving, which they must do to get oxygen.
Jaws contribute a little to the problem too.
What is despicable are fishing tournaments that award for the largest shark caught, which is then thrown out rather than consumed. The most expensive use of shark is for shark fin soup, which is adored by the Chinese. I've tasted it and frankly there are other things I'd rather eat, even spinach and asparagus. One television news report I saw said that some fishers cut the fins off of live sharks and then throw them back in the ocean. Thinking of this, I just couldn't buy shark to eat.