Julie Newmar as Catwoman set alight the nation when she debuted on Batman in 1966. It was the 19th episode and the episode that truly cemented Batman as a true cultural phenomenon. (The Joker had already appeared on 4 episodes before Catwoman was introduced.)
Newmar's Catwoman became the most popular character on the series next to the Dynamic Duo themselves. When season 2 wrapped up, the network was faced with falling numbers for Batman, its highest rated program. How could they get back to number 1 in the Neilsens?
The idea was to have Catwoman as the permanent villain of the week for season 3, which would be down to one half-hour episode a week rather than 2. Not only was this not practical from a storyline point of view, Julie Newmar wasn't available to appear even in a half dozen episodes much less a season of 26 shows.
A pilot for Batgirl had already been shot, considered for a slot for ABC's 1968-69 schedule. But with a combination of falling audience numbers and no Newmar, the network decided instead to fold Batgirl into the Batman series. Hey, she had a tight costume that closely resembled Catwoman's so why not?
Eartha Kitt assumed the role of Catwoman for season 3 when Batgirl made her big splash. Although a very popular character, Batgirl didn't add enough punch to save the ratings decent. Batman was canceled in the spring of 1969 with reruns airing through the summer.
NBC, who were having their own ratings problems, offered to pick up Batman for a 4th year but the sets had already been destroyed so that wasn't financially feasible. Considering how awful most 3rd season shows were, due to a reduced budget, that was perhaps a blessing.
