choosing the right Opto isolator?
Is there something obvious I am missing here? or anything in
  particular that makes the 6N317 quite so desirable for its use here?


According to the data sheet for the 6N137 it can handle data rates up to 10Mbps: -



I took a quick look at the H11A1 and it has rise times and fall times of about 3 us. That informs me that it may be good for data rates of about 100kbps.

That's a big reason in my book for using the 6N137