The first two release was named randomly and came from a joke. And from the third release they decided to use alphabetical order in release names. This is documented very well in the Ubuntu Wiki page

From Mark Shuttleworth's writing, I'm quoting this from the wiki page:

Many sensible people have wondered why we chose this naming scheme. It came about as a joke on a ferry between Circular Quay and somewhere else, in Sydney, Australia:

lifeless: how long before we make a first release?

sabdfl: it would need to be punchy. six months max.

lifeless: six months! thats not a lot of time for polish.

sabdfl: so we'll have to nickname it the warty warthog release

^{lifeless is Robert Collins. sabdfl is Mark Shuttleworth.}

So, the naming convention or policy wasn't fixed before the first release as you might have been assuming. That convention is later introduced and of course it is one of the greatest feature of Ubuntu OS.

• Read the history in Wiki page


• Also read all the dashing naming suggestion given for upcoming releases

Though they decided to use alphabetical name after Breezy, they also decided that they might avoid some letters. I think that is why there isn't a C name after Breezy. The relevant part form the Wiki page is

We might skip a few letters, and we'll have to wrap eventually

And from 17.10 the release names will be wrapped up and will be restarted with A. So, don't forget to suggest your awesome names.