Unable to set DYLD_FALLBACK_LIBRARY_PATH in shell on OSX 10.11.1
This is System Integrity Protection introduced in El Capitan

Documentation is in this from Apple 

Basically any Apple supplied OS X executables are protected. and (from an earlier document)


  Spawning children processes of processes restricted by System Integrity Protection, such as by launching a helper process in a bundle with NSTask or calling the exec(2) command, resets the Mach special ports of that child process. Any dynamic linker (dyld) environment variables, such as DYLD_LIBRARY_PATH, are purged when launching protected processes.


In this case sh is protected