"Nintendo has too much inventory of Gamecubes already. It is not likely that they can sell them all before the next console is released, so they stopped making them."
The Gamecube isn't doing THAT poorly.
The new console won't be out for at least another TWO YEARS. Nintendo will sell the remaining inventory, resume production, then sell A LOT more before the new console is released. Most reports suggest Nintendo plans to resume production around October or November.