ELF supports loading a shared library to some arbitrary memory address and fixing up references to symbols in that library accordingly, including dynamically after load time with dlopen(3).

a.out did not support this. The executable format doesn't have relocation entries, which means every address in the binary was fixed at link time. Shared libraries were supported by maintaining a table of statically-assigned, non-overlapping address spaces, and at link time resolving external references to those fixed addresses.

Loading is faster and simpler when all you do is copy sections into memory then jump to the start address.