You don't spend money advertising a car you're not going to make money on.
What makes money for the automobile manufacturers? Trucks and SUVs by a mile -- then passenger cars. And of the passenger cars sold, the vast majority are sedans like Accords, Camrys, Taurus/Fusions, etc. Sales of any kind of coupe are a tiny percentage of overall car sales -- and performance coupe sales are a smaller fraction still.
Look at Ford. The Mustang is a runaway hit with six figure sales. Yet only a small percentage of those cars being sold have V8s. Even though that car is selling extremely well -- Ford still sells eight times as many F-150s as Mustangs -- with better margins.
To film a television spot on the cheap is $250,000. Air time? Even a spot on some obscure network like Speed is probably $25,000 per airing. If GM's gross income on every GTO they move is $1000 -- and I really doubt they're making that because the units sold is so small -- they're going to have to sell 250 GTOs to cover the cost of the ad and 25 every time that spot appears. And since you need to run a spot for a month or two to generate any kind of awareness -- the media costs add up fast.
The real failure with the GTO was GMNA's positioning of it. Instead of selling it as an Australian built alternative to BMW and Mercedes coupes, a powerful, well-built, great handling car, they sold it as a muscle car by showing pictures of smoky burnouts. That turns off sophisticated buyers in their 40's (the GTO's intended target audience) while bringing 18 year olds into dealerships -- who look at the $30,000 plus sticker, leave, then wind up in a square Scion with a fart-can exhaust.
It is absolutely amazing that this product sold out in Australia, the Middle East and the UK at double the price for years -- yet, due to GMNA's idiotic marketing, it can't be given away in the United States.