In 1970, CBS joined other networks in cancelling rural themed programming that appealed to the mass TV audience in what's known as the rural purge. Instead, the network began focusing on two different and new waves of shows deemed "quality" and "relevance" programming. MTM and its filmed productions with unique and more cinematic aesthetics and lighting designs exemplified the former while Lear's focus on issues oriented programming particularly with regard to race, characterized the latter. CBS made this shift for a few reasons, but most especially because Nielsen changed its ratings metrics to account for a "quality" audience as opposed to just the mass audience. Suddenly and for the first time, the demographics of the audience mattered more than the size, and urban and younger eyeballs were worth more to advertisers than elderly and flyover state eyeballs. The other big change that happened was the passage of two FCC policies meant to boost local productions that stripped the networks of their syndication divisions and barred them from owning the content they played in primetime. Of course they still wielded considerable control, but these temporary policies (FinSyn and PTAR) gave more power to independent studios and cut back on the bottleneck market the networks had created with unchecked power. Because they didn't own his productions but leased them, Lear had creative control over his programming and constantly threatened execs not to deliver his leased content when they tried to censor him too much and that led to infamous battles between himself and the networks which peaked with a first amendment suit he eventually lodged against them. For more I recommend reading the work of scholars Kirstin Lentz and Taylor Cole Miller.