I've read on this website a couple answers to that question. Some that I remember was stem cell research/growing teeth and lack of fluoride in drinking water. Do a search, you should find some good answers.
If I had that question in an interview I would say it is getting people in to see the dentist! Some people w/ dental insurance will not even come in. People need to value their teeth & dentists need to do a better job of getting people in the chair. They risk their oral health for life by not seeing the dentist for 5+ years at a time. That is the challenge & pressing issue of today's dentist IMHO (& sometimes ignorant opinion) .
Since 1960's we have known cavities are caused by bacteria! Only recently has there been a push for having a more medical perspective to attacking this dental disease. It was drill , fill , and bill. Now there is a big push for Prevention! Check out my web site at www.cohpmc.org (well it's not mine but i pretty much update it! When i have time!)