Airline Code
 

Why is Jet Blue code B6 not JB.

Because that code was already taken by Helijet in Canada at the time JetBlue applied for their code. As such, IATA decided to pick a code starting with J or B and since B1-B5 were already allocated at the time, they assigned JetBlue B6.

Just ask WN.

Or NK or G4 or F9 lol.

Every once in a while, codes change.

For years, USAir was AL (from the Allegheny days). But it was eventually changed to US.

I heard this somewhere, but I'm not sure if this is true or not. The IATA in this day and age will assign 2-character codes at random due to the airline abbreviation codes all being taken, but if an airline wants a "vanity" code, they'd have to request it, and that may have costs involved. If you're starting an airline in this day and age, a matching IATA code is probably not very high on your list of things to spend money on.

Neeleman's Breeze is MXY?

That would be an ICAO code (which are 3 letters), which is not the same as an IATA code (2 characters) which is used primarily for ticketing. Both codes are used for flight identification, although I think the IATA one is still slightly more prevalent. For the record, JetBlue's ICAO code is JBU.

I believe that the original plan was to call the airline Moxy.

An improvement over Taxi!