Dust to dust

It's Ash Wednesday, the beginning of Lent.
This isn't going to be a post about Lent. I don't really observe Lent by giving something up or eating fish on Fridays. I just have a question about Ash Wednesday.

Even as a Catholic kid (that did observe Lent and get the ashes on my forehead) I wondered about the ashes. I mean if they are going to say the "dust to dust" line, why don't they use dust or dirt? They don't say "ash to ash." 
Is it just because the ashes last longer and everyone can see that you are a devout Catholic or whatever?
Maybe Dust Wednesday sounds too much like a day for a particular household chore to be done.

I don't really expect an answer. It's just a curious observation.

John