Because it is the first;))))))))

As you probably know most of Russian feminine gender nouns end with "a" and a lot of them - with "я". So, naturally females' names have this kind of  endings.

That would be my take on it.

True.It's same with bulgarians.But it's not only with "a".It could be with other vowel too.

Why not? What letter would you prefer it to be? :)

Ia don'ta knowa... whata doa youa thinka?