This is just random. It's not from Delta, it is just a coincidence that you are a Delta flyer. They didn't Delta's system or get your information from Delta. Spammers get lists of millions and millions of emails from various sources. Then they pick companies which are used by millions of people such as PayPal, Ebay, Delta, United, Amazon, UPS, DHL (...the list goes on) and they craft an email that would look as if it was legitimately sent from the company. Someone can have the "from" line in the email be whatever they want, such as Delta, even though it's not really from Delta.
This is why it is called "phishing". Because they are just throwing out line after line with your bait, and hope that you catch something. It is not necessarily targeted at you because they hacked your information and know that you fly Delta. Even in the most barren part of the ocean, if you throw out millions of baited lines you are bound to catch some fish.