Very simple, it's a free market.
Registrars all pay a $6 fee to the registry for each domain they register and they are allowed to sell domains at any cost they wish.
In fact, the two most popular registrars, Network Solutions and Register.Com, both charge over $30 per year.
As for eNom, I suspect most of their customers do not pay $30 year. Their company caters to many resellers and sub-resellers that purchase domains for as low as $6.95.