I have a feeling it is entirely because of the gender-neutral issue. When referring to a male who serves you food, "waiter" is entirely appropriate. Similiarly, "stewardesses", along with being the longeset English word typed with only the left hand on a standard keyboard, is approriate for female flight attendants. However, when referring to an unknown person doing this job, it is considered (in the US, at least) more appropriate to use a gender-neutral term, as "waitress" does no apply to men.
Example airplaine usage:
For service, please press this button. A flight attendant will come by to assist you.
Saying "stewardess" here implies that in every case, you will be assisted by a female attendant.
Americans place an (excessive) empahasis on being politically correct, and go out of their way not to say/do anything that might offend a stranger, or, especially, an employee. You can sue people for just about anything these days, so businesses are cautious to be as non-offensive as possible.