Because its coded into the protocol and it can't be changed. I'm going to try my hand at an analogy here.
Imagine bitcoin as a little snowball that was rolled down a hilltop in 2009 and it just keeps growing bigger and bigger. The same rules that governed that snowball in the beginning still govern it today.
Someone can change the rules if they want, but then it wouldn't be the bitcoin snowball, it would be a different snowball, and the original bitcoin snowball will keep on rolling undisturbed.
There are many many little snowballs out there today. They are known as "alt-coins". Litecoin, Peercoin, etc. Each on of them is trying to out-do bitcoin in one way or another to attract more people so that their snowball can grow bigger than the Bitcoin snowball, but the problem they are facing is that the bitcoin snowball has grown so big so fast, that no one really cares about their little snowballs since the bitcoin snowball is by far the most established and has the largest number of backers to keep it growing bigger by the day.
I hope that communicated well. If not, I can elaborate further.
Here is a list of the other "snowballs" by market cap:
http://coinmarketcap.com/
Edit: In hindsight, "rolling down a hill" isn't the best analogy. "Being pushed across, flat snow-covered ground by more and more people" is a better one.