The a.out format forced shared libraries to occupy a fixed place in memory. If you wanted to distribute an a.out shared library, you had to register its address space. This was good for performance but it didn't scale at all.
By contrast, in ELF, shared libraries can be loaded anywhere in memory, and can even appear to be at different addresses to different applications running on the same computer (with the code still effectively loaded in only one place in physical memory)
