It's definitely cost. Last year, Apple had a problem with iMacs overaccumulating in the warehouses, partly because they had too many models. They filtered it down at the start of this year to the two models that people tended to buy: the mid-range 15" Combo model, and the 17" Superdrive model.
Apparently that's been working well enough for them that they wanted to keep it going for the next several months.