I believe it is because it is a phrase. The person who would ask for "einen kaffe mit zucker bitte" would be the Nominative, whilst the coffee would be Accusative.
If I said what i wanted (Ich möchte) einen Kaffee mit Zucker, bitte If I asked you (Möchten Sie) einen Kaffee mit Zucker?
Its a phrase, an incomplete sentence, which is why i believe it has no Nominative case as it is implied and omitted.
That is not true.
If you look into literature, chapters tend to begin with indefinate articles such as: "A New Hope", "An Old Friend". Another example would be in media "A car crash on the motorway is blocking traffic northbound..." Or "A new study reveals that smoking is mainly responsible for COPD". Jokes are another one: "A man walks into a bar".