So, years ago we had expansion packs for PC games. These packs used the same game engine and merely added assets, e.g. extra levels/missions, weapons and characters. I'm thinking of things like the Half-Life 2 episodes, Doom 3 expansion, HL1 expansions, WC3 expansion, etc. They were fairly commonplace and usually took 12-18 months of development to pump out.
Today, expansion packs have been split into DLC and entirely new games. Looking at Modern Warfare 1 and Modern Warfare 2, what is so vastly different that could not have been released in an expansion pack? These games are taking 18-24 months to pump out, so basically a big expansion pack. Except with a $59.99 price instead of $29.99.
Can anyone come up with an answer better than merely money-grabbing?