Why is the Hafs reading of the Qur'an so prevalent?

Of everything I've read, the Hafs-an-Asim reading of the Qur'an is used by the vast majority of Muslims (it's definitely the only I've ever seen or heard used in my area). I've even seen articles claiming that this is the case for up to 95% of the Muslim world, with the Warsh-an-Nafi reading holding second place with a whopping 3%. However, it is also my understanding that all seven major *qira'at* are considered co-valid, and are all considered perfectly valid methods of reading the Qur'an. Why, then, did the Hafs reading gain such predominance? Is it somehow more "correct" than the others? Was it promulgated (and the others restricted) via political decree? Is there actually no rational explanation, and it's just...one of those things?