Why was a Lockheed L-1011 chosen by Orbital ATK for its Pegasus launches?
One reason the L-1011 was chosen was that the keel beams on the L-1011 are spaced such that they could create an opening inside the aircraft for the verticle tailfin of the Pegasus when mated to the bottom of the aircraft and do so without jeopardizing the structural integrity of the airframe. This allowed them more clearance than other aircraft although the L-1011 still have to be jacked up to mate the Pegasus.

Second, while the L-1011 is no longer manufactured, there are enough of them still sitting around that getting parts was still an option, and in fact, sometimes entire aircraft were available for purchase.

Third, they were able to acquire their own L-1011 simulator from Delta and they operate it on a regular basis to maintain their flight currency. Since they own it they can make modifications to it to allow them to not only simulate their training environment but also their launch environment. 

And lastly (although may not have been a consideration) some of the pilots of the Stargazer were Lockheed test pilots.

For more information, you should watch this short video:

NASA Edge Program - OrbitalATK ICON Program