[oscargrouch]

For ELF i would say because its easear to reuse a lot of tools that already deal with the format. Debuggers, linkers, compilers, etc..

My guess is also because its easear to have a bridge with Linux executables, so you can run the same ELF and emulate the linux syscalls somewhere like FreeBSD and now Windows .

I bet they were thinking about running Android stuff more easily, and i dont think there will be much more to gain in designing a whole new executable format, when you compare how much of free tools, code and compatibility with Linux you will have in the end.