I think it was because Yoshikawa wasn't a major character in the manga, or at least less so than in the anime. Then someone in production went "Wait...the manga has a Kirino love Sensei plot, but we've been building up the IshidaXYoshikawa pairing! We need to get this fixed up!" So they had her knocked up to make way for the KirinoXSensei pairing.
My theory, anyway.