Isn't it simple? Microsoft is sinking a huge pile of money into improve Bing, getting market share and making acquisitions (altough I guess these wouldn't be expensed directly against income).
Whether or not that's a wise strategy is another matter but there really is no mystery here.
They are not making acquisitions -- They acquired powerset over 2.5 years ago. More than 10 quarters ago. I'm not aware of any other acquisition since then, or anything in the online space besides that in the last, what, 4+ years?
Not that I disagree with that choice, acquisitions just because you have cash is just as stupid as no acquisitions. Better for them to determine a strategy first.
Whether or not that's a wise strategy is another matter but there really is no mystery here.