Interfacing to a differential signal with isolation
There has to be enough current through the LEDs to make the optos work. 

A terminated RS485 driver might have a differential output voltage of 2V, so you would only have about 800uA through the LEDs, which is pretty low, especially if the baud rate is high. 

Of course you can always add a proper receiver and use that to drive a digital isolator (opto, magnetic or capacitive), which will cost more and require a power supply but will work first time up to many megabaud. I know which one I would do.