Does anyone know why /what makes red velvet cakes turn out so soft and moist? 

Hi,

To my understanding, the main factors that make red velvet cake moist are a combination of using acidic ingredients (buttermilk and vinegar) and brown sugar, which is more hygroscopic that plain white sugar. Depending on the recipe you're using, if the fat content in your cake comes from oil instead of butter, that's also a contributing factor.