Because numbers are measurable (duh) and firm. Everyone understands them, and the fact that number X is bigger than Y means they're the single best tool in an arguement, especially since gamers are typically a fickle and petty bunch.
Not to mention that the console that sells better arguably has more money to put back into its services, licenses, and so on. A good year for Sony will translate on some level to a good year for Sony fans.
This. And it becomes a cycle. Initial sales figures are CRITICAL. Microsoft really shot themselves in the foot with their earlier DLC policy, and remember there's nothing preventing them from going back to it. Sony was able to capitalize on their poor judgment and really kick M$ in the teeth on the issue. Unless M$ performs a miracle, their initial sales numbers will be much lower than expected. The damage is done.
Devs don't make games for the best platform. They make games for the platform they think will allow them to sell the most games at a decent profit. Games today are like movies 30 years ago. They can cost over 50 million dollars to make a good one. At about $50 a copy, they would have to sell a million units just to break even.
If the new XBox starts off with weak sales, it will make it impossible for the developers to recoup their losses. Depending on the company, it could run some of them out of business. Can you imagine the free press Sony would get by taking a picture of some unemplyed guy holding a cardboard sign saying, "Lost my job to Miscrosoft's DLC policy"? And Sony would take advantage of that. They couldn't do anything crass like make a commercial that pokes fun at the situation, but they could do something classy like hiring the guy to work at Sony until he could find a better job. It would be easy for Sony to overplay that hand.
And with Windows 8, this couldn't come at a worse time for Microsoft. Yes they have cash reserves, but that's no reason to throw good money after bad. At some point they will be forced to cut their losses. They will have to.
I expect some kind of spectacular M$ marketing blitz to push the initial sales of the XBox One. If not, they could be setting the stage for the biggest epic fail death spiral in recent times.
a. Initial sales are low.
b. A few devs go belly-up just on initial sales projections.
c. M$ does nothing to bail out the devs (not too big to fail).
d. Devs stop making new games for M$. They can't afford to.
e. Nobody wants to buy the XBox because they hear "there won't be any games available".
f. M$ misses the all-important Christmas sales window by still not correcting the problem.
g. By mid-December, people start running the numbers and begin asking if this is the death of Microsoft.
h. All-out panic ensues as companies like Gamestop refuse to sell XB-1 consoles or games.
^^ This is the exact type of scenario that Microsoft desperately needs to prevent. I've yet to see them do anything serious enough to prevent it. But hey, maybe their numbers experts are telling thim it isn't going to happen. Or maybe they simply aren't preparing for worst-case scenario.
I hope I"m wrong here. The presence of the XBox forces Sony to compete. I don't want to see Sony get complacent either. Competition is good for the consumer.