Why electronegativity instead of electropositivity
The first thing that should be highlighted here is that electropositivity is simply the opposite of electronegativity, any of the two can be used interchangeably with the necessary modifications to the sentence. 

It is true though that electronegativity is more commonly used then electropositivity. For example, the Wikipedia article on the subject is electronegativity, and mentions electropositivity as its opposite. I believe there are two reasons for that, related to each other.

First: Consistency

It's very important in the scientific community, from reasons that go from facilitating students understanding to the writing of papers, that standards are used, and that everyone is talking about the same thing. It's easier if everyone is talking about the same thing, even if they are the complete opposite. It just makes us think faster. Why choose electronegativity then?

Second: How we explain chemistry

It's unecessary to explain why and how the study of the electrons "behaviour" in atoms and molecules are important to chemistry. Usually, when explaining a phenomenon, we talk about where the electrons are "going to". We say a reaction occurs because an electron is taken by an atom, or maybe because it's donated by one. We explain the polarity of the H-O bond by saying that the oxygen will attract the electrons of the covalent bond a lot more than hydrogen will, and not (usually) that the hydrogen doesn't attract the electrons as much as oxygen does.

Again, it's obvious that you can explain everything by means of electropositivity, but, in my experience, we usually explain things by saying which atom/group has the property that makes electrons go to them with higher value, rather than saying which atom/group has the property that makes electrons leave them with higher value.