Because the "red gene" is the same peeling gene that comes from their amphilophus ancestors. Said red gene is dominant by the way.
I don't know where you're going with the jack dempsies only have bars bit. They have black underneath their blue, or at least darker colors idly. Some examples of variation in jack dempsies:
And then some carpintis with varying amounts of blue (you can see how the pattern underneath changes as the pearling expands)
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As for your bit about fry, EBJD fry look nothing like cuban fry. Cuban fry are grey and pointy with that little racing stripe once they start looking like fish. EBJD fry and normal JD fry are both fat and have that weird marbled pattern, and are indistinguishable until they start to get blue. You could argue that that's just because the fry are getting their baby color genes from JD, as in other crosses I've experienced such as cutteri to HRPs the fry take their baby coloration from the cutteri with slight body shape differences, but it wouldn't make sense for EBJD fry to look exactly the same as wild JD fry.
It's not parroting information if I can comprehend and explain what I'm saying.
