[Look the Statix III is a 65nm chip. Alterra isn't even making it anymore. They want to sell Stratix IV and soon they will need to sell Stratix V. They want that inventory gone. Gone from their website, gone from wholesalers, gone from retail outlets. when Stratix V is in full production it becomes the high end part and the Statix IV becomes the value segment. The Stratix III is just a third wheel.
I'm not sure it works that way, though - there's always customers out there who've built designs around older chips and for whatever reason either can't or don't want to move over to the latest and greatest. From what I've seen the FPGA vendors are really slow at taking older chips out of production for this exact reason.
They are slow which is why the Stratix III is still around despite being released almost 5 years ago. Still you notice Stratix II isn't available. Companies tend not to like to keep 3+ generations going at the same time. As the Stratix V nears volume production the company will try to transition customers to the newer products. For large customers they will even provide incentives, demo units, and design assistance to move their bitstreams to newer products.