Delta-air.com was changed to Delta.com to be in sync with their new brand campaign that rolled out earlier this year. If you think about it, when most people refer to the airline they say "Delta" and not "Delta Air Lines".
And the change makes remembering the airline's web address a snap. With any business, it's the company brand that people usually remember. The name change makes it easier for customer to recognize the Delta product (flights and fares)