Siri started as a beta that was only in the US so why are you complaining, it will come to you.
The difference is that the USA is Apple's dominion, that is where most of their customers are. This is not the same for MS. WP is pretty much a joke in the US mobile world (just go to some carrier shops and say you want a WP, everyone will try to talk you out of it.
Another point is that Apple launched Siri when they already had a very strong marketshare and profit. Again, MS can't afford to make these kinds of mistakes when they are struggling to gain marketshare. Siri and Google Now are available globally right now, and that is not the case for MS. So if someone goes to buy a phone outside the US, they will see okay, WP doesn't give me an assistant but iOS or Android do, and obviously they may prefer those platforms if this is an important feature. MS will not get anywhere in the smartphone market if they keep doing this,