There are some idosyncrasies in the whole deal.

A couple more are that the three letter codes for airports in the US can't begin with K, W, N and Z. Airports within 200 miles of each other must have more than one different letter in the sequence. That is why IAD is not DIA -- DIA and DCA are too easily confused on handwritten baggage tags. The other is that I BELIEVE in order to have an all letter identifier (you can have numbers too) that you must have weather reporting on the field.

FWIW, all of the Air Traffic Control "Centers" (like Chicago Center, Los Angeles Center) all begin with Z, but the terminal radar facilities do not. For those that want to really stretch your brains...

There are some really easy Center identifiers, like:

ZLA -- LA Center
ZDC -- Washington (DC) Center
ZNY -- New York Center
ZFW -- Fort Worth Center

Some almost as obvious ones:

ZOA -- Oakland Center
ZME -- Memphis Center
ZMA -- Miami Center
ZTL -- Atlanta Center
ZJX -- Jacksonville Center
ZID -- Indianapolis Center
ZHU -- Houston Center

But how about these?

ZOB -- Cleveland Center
ZAU -- Chicago Center

(In the contiguous US, there are 20 centers, FWIW)