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One possible challenge is that most eyesight, human included, is generally more responsive to movement than to colour.
Movement can be very distracting or, to put that another way, very attention attracting where it exists in an otherwise fairly consistent visual feed.
Light boxes can only offer one mode of communication as they are currently set up. Conversely a human wielding a red flag (or one of his colleagues) at least has the option to make movement that might attract the attention of a driver who has not noticed a flag. Whether the flag SHOULD be stationary or not may not be terribly significant at that moment.
Inanimate objects do not have that benefit.
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