The initial carrier was NASA's DFRF B-52-008 – aka Balls 8. As the rocket increased in size and weight, Orbital needed a more capable carrier.
A detailed study considered the B-52G, 747, DC-10, and L-1011.
A study was initiated in late 1991 to identify the optimum carrier aircraft for long term Pegasus launch operations. Some of the aircraft considered included the B-52G, Boeing 747, DC-10, and Lockheed L-1011. Some of the factors considered included performance capability (altitude and speed capability for launch), aircraft range (both ferry and launch), modification complexity and cost, aircraft availability, acquisition cost and operational costs.
Following a detailed trade study, the Lockheed L-1011 was selected for conversion to serve as a Pegasus carrier aircraft. Orbital Sciences acquired a L-1011 aircraft in May 1992, modifications to carry Pegasus are complete, and the aircraft is currently undergoing certification testing. The L-1011 is scheduled to be operational in the Fall of 1993.
Source: Pegasus XL Development and L-1011 Pegasus Carrier Aircraft - by Marty Mosier and Ed Rutkowski - Orbital Sciences Corporation - 1993.
Is there some performance reason?
For a fixed payload, nothing stands out in the L-1011 that a 747 or DC-10 can't do. So the reason would have been likely to do with the "modification complexity and cost, aircraft availability, acquisition cost and operational costs."
The aforementioned detailed study is not in the public domain as far as I have looked, but that particular L-1011 has been with Air Canada since 1974. The L-1011 was also not in great demand by the airlines – its slow initial sales due to development delays gave the DC-10 the upper edge – so it would not have been expensive to acquire.