Because difficulty going up without BTC price going up will cause the payback to be less then the price of the equipment. Our next difficulty is on track to be more then a 50% increase again and it is happening sooner then 10 days. If you repeat that just four more times mining is not very profitable.
I just got my last 6990 card. It should make some acceptable money in the next month. After that profit is unlikely. I will turn my equipment off a little before it becomes unprofitable due resale value of the equipment.
Please define what is "profitable". Or what is a good ROI for a business. 20% a year ? 30% ? 60% ?