ABC horribly overplayed its hand with "Batman" when it went to two nights a week with it. The show probably died a season or two before it should have.
Also, outside of "Monday Night Football," which premiered in 1970, prime time remained a disaster area for ABC until the mid-'70s and "Happy Days." It was only then that ABC began to lose its "third network" image.
Well, remember--"Batman"
premiered as a two-night a week show. The idea was the "serial" nature would play better if you didn't have to wait a week. They started in January so that was a half-season, did the full '66-'67 season as two nights a week, then went to one night in the third season.
All told, that was 120 episodes, or four seasons' worth played once a week. That's a pretty good run for that kind of show, and times were changing quickly. "Batman" got the ax roughly the same time (Spring '68) "The Monkees" (58 episodes), "I Spy"(82 episodes) and "The Man From UNCLE"(105 episodes) did.
After "Batman", though---its only real hits were "Bewitched" and "The FBI", both of which pre-dated "Batman". After that came "Marcus Welby", "The Mod Squad", "The Partridge Family", "The Rookies" and "Kung Fu"---but that was it, spread over six years until 1974 when "Happy Days" and "The Six Million Dollar Man" hit big and started the snowball that would eventually include "Laverne and Shirley", "Charlie's Angels", "Baretta", "Three's Company", "Welcome Back, Kotter", "The Bionic Woman" and "Barney Miller".
As I was looking this up, I was surprised to learn that "The Brady Bunch" never made the Top 30 ratings for any year of its initial network run. It took off in syndication, but was never a smash for ABC.