Just to add to gebx's point for argument's sake ...
Although some of those games were bundled Activision would still have received money from Microsoft (or the retailer who bundled the game) which would result in Activision's books showing Microsoft was a platform with far higher sales of the same game. While we don't know for sure, it is rumored that Microsoft's licencing fees (and charges for downloadable content) are lower than Sony's which could translate into higher revenue generated per unit sold.
When you combine payments for bundling games, higher unit sales, and higher revenues per game sold it is entirely possible that Microsoft's portion of revenue for HD console sales is dominant enough to question the value in producing a PS3 version; in particular when you are talking about games with lower sales where the revenue generated from the games' sales barely cover the porting cost.
