Perhaps because it is vegetable in origin, as opposed to animals hooves.

I am not sure if it is generally preferred, but as a vegetarian, I avoid gelatin: "Vegetarians usually avoid gelatin and use other emulsifiers instead, such as agar, carrageenan, pectin, or konnyaku. There is no vegetable source for gelatin." Another reason for avoiding gelatin would be safety concerns: "Due to Bovine spongiform encephalopathy (BSE), also known as "mad cow disease", and its link to new variant Creutzfeldt-Jakob disease (vCJD), there has been much concern about using gelatin derived from possibly infected animal parts. One study released in 2004, however, demonstrated that the gelatin production process destroys most of the BSE prions that may be present in the raw material. However, more detailed recent studies regarding the safety of gelatin in respect to mad cow disease have prompted the U.S. Food and Drug Administration to re-issue a warning and stricter guidelines for the sourcing and processing of gelatin to reduce the potential risk posed by Bovine Spongiform Encephalopathy from 1997." Source and further information: http://en.wikipedia.org/wiki/Gelatin Further information: http://en.wikipedia.org/wiki/Agar