The more it sells, the more they lose. The percentage of market share they may gain by doing this in the short run won't justify the loss.
If only that were true...
THis early in the generation, if the gap between the systems total sales gets any bigger, M$ knows that they are done. Like everyone comments around here a lot about, when everyone's friend has a PS4 - everyone else will want a PS4 to play all of the games with. M$ knows this all to well. So, they are dropping short term profit for long term viability in the console market. If the XB1 can get to parity with the PS4 in the US, that will be a win in the long term for M$. Of course, they will try to actually push to get a sales advantage over time in the US.
Based on the fact that they have NOT extentended the promotions throughout the world, they have decided to not take a total loss.
It is near the end of the end....