Why are not all animals oviparous? Does vivipary offer any advantage to organisms?

All animals are not oviparous as being viviparous certainly offers great advantages. For example certain developments occurs before birth in vivipary, parents does not need to take care of the young ones all the time as they can leave their nest (or home), less offspring can be produced as opposed to oviparous where large number of eggs should be laid to ensure maximum survival, food and oxygen are readily available to the offspring.