Because, as with all dictionaries except the absolutely massive complete editions, you can only put in so many words (this particular volume has 6,000 entries a

Because, as with all dictionaries except the absolutely massive complete editions, you can only put in so many words (this particular volume has 6,000 entries and is aimed at 7-8 year olds). In a related Guardian article somebody from the OUP says the selection criteria are: "acknowledging the current frequency of words in daily language of children of that age; corpus analysis; acknowledging commonly misspelled or misused words; and taking curriculum requirements into account". That is, times change and the things children are writing and reading about also change; dictionaries tend to follow, not lead.