The reason they made Broken Steel was because the game mad you load up your last save rather than let you continue on after the ending. Also, someone else pointed this out earlier in the thread but it didn't require a bunch of effort to change the ending so that you survive no matter what. To change the Mass Effect 3 ending it would take a lot more effort and money and if the ending ruined any enjoyment you had in the franchise you probably weren't enjoying the series that much to begin with.
Come on you all, He's right you can't be a fan of the Mass Effect series if you don't think the ending is bad. Oh yeah and also I'm being sarcastic. The ending isn't great but that doesn't mean it needs to be changed and these people petitioning for Bioware to change the ending are being dumb.