Generally because someone has already got the code the airline would have preferred - by using only a two character identifier, IATA greatly limits the number of airlines which have codes, and its kind of first come first served - so all the legacy airlines have the codes they wanted, but most of the newer airlines have to use anything which is left, which may or may not be anything to do with their name. Codes get recycled too when airlines go to the wall, there is such a shortage of them! Can make looking at historic data interesting if airlines are identified by IATA code, as you have to remember what years some of the major airlines died.