Delta **DOES** have its own callsign. It's "DELTA".

There are literally thousands of airlines in the world, many of which have designated radio telephony callsigns. Some of them are very close or even identical to the airline name, others are more diverse.

I can only speculate here, but the reason British Airways uses "Speeedbird" is probably because the callsign "British" was already taken - it is the callsign of "BIH (Offshore) LTD.".

Another example is Norwegian. They use the callsign "Nor Shuttle" (and a few others), probably because the callsign "Norwegian" was already in use - by the Norwegian Air Force.

A full list of airline callsigns is available in [ICAO DOC 8585][1].


  [1]: http://www.carsamma.decea.gov.br/cms/wp-content/uploads/2018/08/DOC8585_182-orig.pdf