The key to every long-running television show in the history of the medium is, above all else, consistency. Why is Days of Our Lives still on the air? Because although it delivers ridiculous, trashy, mostly meaningless storylines (there's no such thing as real death in soap operas, only the opportunity for a "shocking" resurrection later on), it delivers it with consistency.
So while shows like Community and Dead Like Me had to scrape and claw for every season (and often every episode) to be put on the air because of their unusual storytelling sensibilities, a show like The Big Bang Theory is bound to run on an endless network broadcast for all of eternity because it found an easy formula to replicate, and is more than happy to deliver the same familiar product over and over. The easiest formula to ensure a long and prosperous run? Insert boobs. Aaaaaaaand repeat. That's it.
That's the simple key to success that a solid handful of the longest-running television shows in history have latched onto. Some of those shows, however, may have taken that notion a bit too far, deciding that this element would be the utmost priority, seemingly filling up their production crew with dozens of lighting experts (to better accentuate the bazooms) in lieu of a writing team. Because no one cares about the stories or properly-worded dialogue when they're staring into the warm embrace of boobies, apparently.