single-camera sitcoms. Multi-camera sitcoms (Big Bang Theory, Friends, Seinfeld, Cheers, Frasier, Fresh Prince) are almost always filmed before a live-studio audience, so it's more like a play, the actors have to give the audience time to react and play off that. It's also why these shows tend to have the vast majority of their scenes on the same set. Think the apartments in Big Bang, Friends, and Seinfeld for example. Multi-camera format dominated the 80's and 90's.

The 2000's saw a shift to single-camera format which is filmed in a cinematic style instead of basically recording a play. There's no audience so there's no need to give a pause while things quite down. Single-camera sitcoms examples are The office, parks and Rec, Modern Family, and Malcolm in the Middle. These tend to use on location filming instead of sets, for example they just bought a house to use in Malcolm in the Middle instead of building sets open to an audience like Friends. There's no audience and therefore no need to allow the audience to laugh before moving on. There's a lot more variety of sets in single-camera shows due to the on location filming they do.

Laugh tracks aren't added after the fact they're part of multi-camera filming. They're in less shows now because less shows use multi-camera. In fact, now that Big Bang is finished I'm not sure there's a popular Multi-camera show in production anymore (Big Bang was considered a bit of a dinosaur due to the multi-cam filming).