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 struc…
I think this is part of it. The people who would have to write/design the visual programming system think that it is for losers and are thus not interested in working on it. Most hardcore coders I have met are quite proud of their chops and kind of brush off those who can't cut it (which is fine imo).