Well, here are a few reasons:

- DL is the only one offering ATL-LHR (BA is keeping ATL-LGW)
- There is so much competition on JFK-LHR (AA has 5-6 daily, BA has up to 7 daily, VS has multiple as well, etc.)
- ATL is obviously a stronger hub for DL than JFK

However, DL needs to be in the JFK-LHR market if it wants to be a credible force in the NYC business travel market. It's incredible how much business travel there is in the NYC-London market. Even DL has managed to sell out the BE cabin on tonight's JFK-LHR (of course Sunday night is prime business flyer night but given the amount of capacity in the market, it's still quite amazing to see flight after flight no matter which airline being sold out in J tonight)

As to why there is only one ATL-LHR versus two JFK-LHR - DL has very limited slots at LHR right now and to start making an impression in the brutally competitive NYC-LON market, they have to have more than just one flight a day. DL has ATL all sewn up so they can do whatever they want whenever they want - they will still have the market. I wouldn't be surprised to see ATL-LHR become a 767-400ER within the next year though...