The Monte Carlo had declining sales over the last few years of it's production. For a number of reasons.
First, it was a dated design. The same body style being used from 2000 to 2007. Only some updated interior work, a FWD V8, and the front clip styled after the Impala in the final two years.
Secondly, the market had shifted away from "luxury sport" coupes. Four door RWD cars, with optional high performance packages, had taken over the market.
The 6th generation (2000 on) Monte was designed as a FWD. It was in need of a complete re-design. For both, looks and ability to work as a RWD. Sales were already bad enough. But with the Camaro scheduled to come out, sales of the Monte would've declined even more. So the decision was made to dis-continue the Monte after the 2007 model year.
But many Monte enthusiasts are hoping that the Monte will return some time in the future. Hopefully as a RWD V8 sport coupe that will live up to the reputation it had in the 70's and 80's.
It would've been kinda nice to see a re-designed Monte compete head to head with the new Challenger. But it's more fun to see the Camaro SS kicking some Challenger butt.