Written With Ratings as Flat as Brady Bunch Roof by Bridger Cunningham

The 1973-74 season was a harsh climate for television shows as everything below 43rd place was cancelled, save for movie nights. On the sitcom front, the genre comprised one-third of primetime shows. Sadly, only 9 of the 27 entries returned the following season. CBS swept the top of the chart with 6 of their laugh factories returning as they landed in the top 20. ABC handed out two golden renewals to only 2 shows -- new sensation Happy Days, and aging Odd Couple (the origina, not the dreadful 2015 revival). And NBC, who fared the harshest cancellation fates this season, only spared Sanford and Son. Good Times and Happy Days were the only freshman sitcoms of the 12 offerings to return.
10 Freshman sitcoms failed, 2nd season Temperatures Rising and The Brian Keith Show never survived ABC's chopping block, and CBS's Dick Van Dyke revival turned old with a dismal 3rd season. The remaining 6 shows axed possessed long runs worthy of syndication. CBS turned Lucille Ball's last successful sitcom, Here's Lucy, blood red after 6 seasons. ABC tossed a nickle severance to Brady Bunch, Love American Style and Room 222 after 5 seasons, and the Partridge Family's disastrous last season ended with four fingers into the rerun glass. 1974 ended the flat-housed technicolor era and turned laughs serious via social relevance on CBS.
Source -- http://startrekfactcheck.blogspot.com/2014/07/the-truth-about-star-trek-and-ratings.html