most schools want a letter from science professors because after all, dentistry is a science and they want to know what type of a person you are through a science professor's point of view. all of the professor in dental school are science professors and so that helps the most.
some school want letters from workplaces/volunteer officer/etc. they want to know you from a work experience point of view to see if you can get a long with people from the workplace. they want to see if you show up on work on time or your the type that slacks off and comes late to work...etc. With science professors, they can't explain all that.
They can also request other letters from other people to see the complete picture of you. thats the reason behind letters of recommendation.
so choose wisely of the people that writes your letter. choose the ones that describes the type of person that you want dental school to know about you.