When it comes to biology/evolution, to ask the question "why" is unanswerable. It's not the way things work when it comes to evolution. Things are the way the are now because of millions of years of selective pressures dictated all current living things into their current constructions.
In fact, the question "why" is nearly impossible to even consider when it comes to biology. Things are the way they are not for any particular reason. Maybe there were organisms that had developed a large "blowhole" like structure that was at the end of their nasal cavity. But for whatever reason, which may or may not had anything to do with the having two small nares or a bigger blowhole, environmental pressures allowed one organism to succeed and pass along their traits (including the nairs) and the other became extinct.
We can talk about structure's function that it provides, but that structure was in no way "designed" that way to provide that function. In fact that function may or may not have been essential to that organism surviving and passing along that trait. Some traits are simply along for the ride. In those cases it's other structures that provide increased fitness which allows certain organisms to outcompete and prosper through evolutionary time.
In the case of having two nares at the opening of the nasal cavity, I have no idea if this was a critical structure for the propagation of organisms on earth. I can make the guess, however, based on the fact that many animals do have similar structures, that this feature was present early on in evolutionary time. The fact that this trait has been maintained suggests one of two possibilities: either these structures do have some level of importance for organisms' fitness or 2) there is no real pressure otherwise that has forced a change from this trait that developed early on.
If there have been some insight gained which is true, I'm not aware. Hopefully someone who has that expertise will find your question and answer. In the meantime, I hope you will consider why the word "why" is not an appropriate place to start the path to finding out.