"Why is it taking so long?"
1. They are starting with "a clean sheet". Where as the Challenger already had a chassis (the 300C), allowing it to arrive earlier, the Camaro is all new.
2. Chevrolet is taking its time to make sure what arrives in production form meets all the improvements in quality, etc..
ALOT rides on the success of the Camaro, and rushing it to market, only to have owners, and magazines, giving it negative reviews would not bode well for Chevrolet.
As "our leader" Fbodfather keeps repeating, "Have Faith", and a little patience...it'll be well worth the wait.