Sitting young children in front of a television for hours also prevents that child from having hours of other developmental experiences. On the other hand there are many positive qualities to modern technologies. The technologies that benefit young children the greatest are those that are interactive and allow the child to develop their curiosity, problem solving and independent thinking skills. I think the difference between computers and television is that television tends to be quite passive. With television, they watch and do not control anything.
Computers allow interaction.
Children can control the pace and activity and make things happen on computers. They can also repeat an activity again and again if they choose. That's actually a really good question. Preschool children are still having significant cognitive growth. In a very real sense, children think differently than adults.