Why Not all Poisonous Frogs are Poison Dart Frogs

The popularity of such beautiful creatures as poisonous frogs made them very attractive to the hobby, and as such, they are nowadays among the best known frogs in the world, both scientifically as by herpetoculturists.Even the comprehensive treatment of poison frogs (Lötters et al. 2007) fails in describing what a poison frog and a poison dart frog are. So the question is, why name a whole family with a feature that is only common to some species? And why is so popular to name all representatives of Dendrobatidae as poison dart frogs?