They've been using the same engine in every CoD game since CoD2. Also, MW2 was a mega sucess. Why would they completely change everything and possibly make it worse?
All they've done is added a bit more content, the continuation with the campaign story and apparantly fixed the imbalance in the multiplayer (I personally didn't see any imbalance in mw2).
Now i'll be the first to admit that the map packs are a complete rip off, but people will still buy them up :/ Nothing you can do about that.