Probably because they started using engines from other divisions in their cars. They got sued by a bunch of Olds & Pontiac owners who were pissed because they got lowly Chevrolet engines in their cars.
Maybe chevy decided no wimpy mid 70s 80s smog motor should bare the same beautiful Orange that its High reving Hi-po motors from the glory days wore.
Or may be some unemployed envirotard from california decided that Orange was bad for the ecosystem.
Or maybe chevy motors last so long between rebuilds that the Orange paint comnpany hadto discontinue the color for lack of sales of spray cans.Forcing chevy to use a lightened shade of Ford blue.(Im gonna hear it now LOL)
Or maybe the company that made the orange paint was from NY and GM could nolonger afford the ever-rising sales tax.
Or maybe some money grubbing executive in a suit at GM decided that bluemotors would apeal more to the public making it possible to raise the price of each model.This would be my guess.