IIRC because the L-1011 used plug doors for the cargo hold, unlike the 747 and DC-10 which used outward-opening doors
Another example:
All three L-1011 cargo doors (C-1, C-2 and C-3) opened outward!
The C-1A cargo door (used on all -500's, all BA -200's and the two CX -1's obtained from Court Lines) was almost identical to the forward cargo door of the 747, also opened outboard!