Answers to this are numerous. A useful object of meditation should be one that promotes mindfulness. It should be portable, easily available and cheap. It should also be something that will not embroil us in those states of mind from which we are trying to free ourselves, such as greed, anger and delusion. Breathing satisfies all these criteria and more.
Breathing is something common to every human being.
We all carry it with us wherever we go. It is always there, constantly available, never ceasing from birth till death, and it costs nothing. Breathing is a non-conceptual process, a thing that can be experienced directly without a need for thought. Furthermore, it is a very living process, an aspect of life that is in constant change. Thus it is a miniature model of life itself.