It's the way the theory was created, really. And again, it's a theory, so until there's conclusive proof in an evolutionary way, it's all discussion. But from what scientists observed, this is how they see function order occurring. You can't really be extroverted and introverted at the same time; one wins out, because we have to function that way. We're either thinking in our heads or we're speaking out loud. So when the functions occur, you have to do either one or the other - you can't do both at the same time. And you have a particular preference for the one you do most often.
Now, that doesn't mean you *can't* do another another preference; we do use all of our functions. But we have an order that we prefer. It's uncomfortable - and unnatural - for me to be an extroverted individual, but I can do it if I have to.
If it's better, don't think about it as if you can't do it; think about it as if you don't want to do it. You just don't like behaving a certain way, and you've naturally become the person you are based on the patterns of your mind. Behavior is the outward manifestation (theoretically) of your preferences.