I'm surprised they weren't in from the start, given that they're visually identical to githyanki. Maybe it's because shield and detect thoughts aren't implemented yet? And/or because there's no monk yet (given that their philosophy makes monk essentially their favoured class).
I think it might also be because yes, they're visually identical or almost identical to githyanki, but would have completely different dialogue. Lots of writing would need to go into githzerai. They'd need to react to illithids, to surface creatures, to githyanki, all in a githzerai way that's radically different from githyanki. I also think it'd be harder to write githzerai so they feel githzerai (and not humans in githzerai guise), moreso than it is for githyanki. (Unless later editions diluted their flavour, my githzerai knowledge is mostly from P:T.)
