@barkerg has some of the best Greens in the US, there are a few others as well that are not well known but in general, you are correct. Nutrition, pen space, and inbreeding from a limited gene pool has affected the birds in the US.
The commercial feed industry has ruined peafowl in the US as they have been treating peafowl like they do meat birds feeding them like you would fatten birds for slaughter. The use of soy and corn because it is cheap protein is the worst culprit in the diet. Peas need a meat/fish based protein diet that also supplies the green roughage for a balanced diet.
I think another factor -- especially with the IBs -- is natural selection for body type that suits colder climates than that in which the original species is found in the wild. Where birds experience Winters, a more rounded body type will be selected over time because it favors heat retention. Lanky, leggy birds would be more prone to heat loss in cold climates. I'd wager that if someone imported wild IBs and kept them where Winters happen, they'd need to be given accommodations similar to those recommended for Greens and high percentage Spauldings.
I feed all of my birds a mixture of game bird feed (designed for feeding pheasants and partridge for release) cat food, meal worms, a little cracked corn in winter, they also get whatever veggies we don't want. I have noticed in just one generation my birds have gotten leggier and more slim so I'll keep on breeding for that on top of working on making their nutrition the best I can find.
I think reckless breeding in general is destroying the peafowl hobby more than bad nutrition ever could but bad nutrition definitely plays its part.