Spreadsheets show us that 2d grids and a lack of other programming concepts make it very hard - it's a distinct subset of flow programming:
A unit of code an take an arbitrary number of inputs from a 2d grid and give you 1 output. Real Flow programming lets you have multiple inputs and outputs and aren't constrained to 2d. I think the biggest constraint is representing the dimensionality and interaction ... but we have classes and deep package structure in Java, so it's probably more of a effect than cause, but still a blocker to wide adoption.
A unit of code an take an arbitrary number of inputs from a 2d grid and give you 1 output. Real Flow programming lets you have multiple inputs and outputs and aren't constrained to 2d. I think the biggest constraint is representing the dimensionality and interaction ... but we have classes and deep package structure in Java, so it's probably more of a effect than cause, but still a blocker to wide adoption.