"The story, as far as I've been told and learned, goes as follows; Infinity Ward wanted to take the COD series more modern as early as COD2, but Activision wanted to continue to compete in the safety of the WW2 shooter market and made them keep the series in that setting, promising them that chance with COD3.
COD3 instead got handed over to Treyarch, which made IW really angry and demanded a revised contract that gave them exclusivity over numbered COD games. This is why Treyarch cannot make COD5 or 7 and instead World at War and Black Ops.
COD4 was more or less their magnum opus. They had no intention of continuing the series, in fact MW1 ends very much on a pretty final note. However Activision wanted to continue the success of the series and with-held bonuses that were to be paid upon the completion of MW1, ransoming their paycheques for MW2.
MW2 was played very close to IW's chest. No open beta, no external testing, Activision didn't even get to see the game until it went gold. They make every effort to avoid actually calling it 'Call of Duty: Modern Warfare 2' except on packaging and promotional material Activision has control over.
If you ask me, I think they just took a laundry list of complaints and suggestions from a bunch of 14 year olds and squeezed them all into the game, put some shooting action to a Michael Bay movie and called it a day. They tested to make sure the game didn't explode when you put it into your sytem. "