I don't know why (I'm guessing their marketing people have had enough to do in launching bmibaby and the new one way fare structure), though I would expect them to launch this soon, as well as self check-in machines. They already have the technology (when they changed over to LH's reservations system last year they gained the ability to handle e-tickets).

In fact, it seems possible to book LH codeshare flights (operated on BD aircraft) over the LH website, and receive an e-ticket. I have never actually done this though as LH is usually slightly more expensive than BD on these flights.