User blog comment:Erockrule/Zatannas joining the team/@comment-99.74.206.12-20111029165435/@comment-3465526-20111029171802

No, they wanted to use Wonder Girl when they wrote the series last year, but back then there was a whole tangle of legal rights issues around the character (Wonder Woman and derivative characters are all tied up (so to speak) in a weird contract that goes back to the first deal with Moulton the creator of the comic)

Those legal issues were worked out, but not before they had done so much work on all the episodes that they couldn't put her into the first season if they wanted to -- I fully expect she'll join the Team in Season 2: INVASION (scheduled to begin in March)