Because it is a crock of shite, that's why. Ask anyone who ever used an actual (general-purpose) "visual programming system" for any (nontrivial) real life task. I have extensive experience in one, and a little in another, and both suck balls. The problem is that, while the diagrams you are constructing might look fancy and intuitive to a layperson (i.e. your pointy haired boss), they do usually not help you to structure the complexity of the task at hand in any meaningful way. Usually, they make everything worse. Having said that, there is a rather big exception: Domain specific visual programming systems, if they are well designed and the problem domain lends itself to it, can be quite useful.