Those statistics may be misleading. Some people only practice vegetarianism for particular times such as fasting for religious reasons or financial reasons. Many of my non-vegetarian family members didn't eat meat while flatting at university to simply save money.
Other people stop being vegetarian once they leave their parents home or travel abroad, particularly international students from India. Some see eating meat as cultural assimilation in Western countries and even remain vegetarian when visiting their family. If they're raised vegetarian rather than choosing to do so I'm not sure this is "quitting".
Another consideration is that ovo-lacto vegetarian is assumed for vegetarians so many people on a stricter diet may not identify with the term. Does becoming vegan mean they wouldn't be counted as vegetarian here?
I wouldn't consider many of these people to have "tried" or "quit" vegetarianism. I'd be very cautious of interpreting or designing these studies due to potential confounding variables.