there are many possible reasons for that:

• she might have thought that the general critic on gender roles is allready enough for the reader
• she saw singular "they" as something for written or formal language and not everyday talk, which is what most of the dialogue in the book is.
• she was not happy with "they" since it can lead to confusion around singular and plural or did not see it as better alternative than a generic masculin pronoun for someother reason
• she simply did not use "they" that way when she wrote the book.

I sadly don't own the anniversary edition, so i don't know if le guin elaborates further on that. It's definitely valid criticism however.

e: english grammar hard.