I think at least part of it is image. "real" programmers dismiss visual languages, especially ones aimed at kids like Scratch or Snap as toys, not the kind of thing Real Programmers use. (insert references to all the Real Programmer humor)

Aside from that, there is the issue of tooling (source control, etc), editing large blocks, etc. which the visual languages I've used are not great at.

But it should be recognized that some things are better visually and some things are better textually. Typing "a = b + c" is way simpler than dragging a bunch of blocks around to describe the same thing. But visual tools are superior for understanding relationships - a connects to b, which connects to c makes a lot more sense when you see it as "[a] -> [b] -> [c]", and an ascii diagram like that quickly becomes unwieldy while graphical boxes still work.

I find an interesting comparison between drawing diagrams with a diagramming tool (e.g., Lucidchart) vs with a textual description language (PlantUML). I find the textual language far easier to use to quickly produce diagrams, but LucidChart is superior for tweaking the exact dimensions and alignments of things.

All of which is to say, both approaches have cases where they work better, and others not so much.