This is a long post, but it should answer the OP's question.
Probably because of the age of those posting here, most of you are acting like the 90s were the golden age of sitcoms or something. But when you look at the overall percentage of viewers watching TV across all channels (and eventually internet/streaming as well) and genres, sitcom popularity has been dropping since the late 70s/early 80s.
1980s
Cable TV - Viewers went from 3 or 4 choices of shows to dozens due to cable television in the early 80s.
Children (and their families) watched Nickelodean. Teens & young adults went to MTV. Older adults started watching CNN and the History Channel. Network television saw losses in all age demographics. Multiple sitcoms were in the 30s on Nielsen ratings in 1979, but the highest by the 1981-1982 season was 23.34. If you're doing the math, that's a 20% drop in just 3 years.
78-79 season: Top 5 sitcom avg = 29.10 rating. Top 10 avg = 27.04 rating
81-82 season: Top 5 sitcom avg = 23.06 rating. Top 10 avg = 22.03 rating
Every sitcom in the to 10 of the 78-79 season scored better than the #1 show just 3 years later. This wasn't a fluke; it was the "new normal" (damn I hate that phrase, but it fits here).
1990s (specifically starting in 1992)
Continued Growth of Cable - Now 100+ channels. Network TV loses viewers every year in every genre.
News Magazine TV - During the 1992-1993 season, real time coverage and opinion-based discussion of the LA Riots exploded on cable TV. Covreage of the OJ Simpson trial, Menendez Brothers trial, Monica Lewinsky scandal, John Wayne Bobbitt, & celebrities in general took the ratings of low-budget startup cable channels like CourtTV and through the roof. The number of "News Magazine TV" shows grew by over 1000%, and so did total viewership. To compete, network channels replaced sitcoms with shows like A Current Affair, Dateline, E! News, & Extra. Many aired multiple times a week on multiple channels in syndication and had numerous spinoffs. Many were an hour long, so they replaced 2 sitcoms each time they aired. They cost almost nothing to produce, and the ratings were almost as good as sitcoms.
92-93 season: Top 5 sitcom avg = 18.28 rating. Top 10 avg = 15.57 rating
98-99 season: Top 5 sitcom avg = 13.94 rating. Top 10 avg = 11.84 rating
For the record, Nielsen tweaked how they calculated ratings around 1990, so these scores are actually much lower in comparison to earlier years than they appear. Once again, however, every sitcom in the top 10 in 1989 had more viewers than the #1 show of 1999.
2000s and beyond
DVR - Because of the use of DVRs, the ratings now have countless categories and methods of scoring. Watching live, day of viewing, live +7 (live or within a week), online viewership of network shows, etc make it tough to compare to previous seasons. The overall scores in Live +7 compared to previous ratingswere half of what it was in the 80s across the board in all genres. Revenue also took a big hit because sponsors paid less for L+7 since commercials were being skipped.
Reality TV - Survivor was the first modern super-hit, but reality shows have been around since TV began. Road Rules on MTV gave networks the idea that it could work in mainstream TV too. Between 2000 and 2005, the number of reality shows, competition shows, and primetime game shows grew from nearly 0% to nearly 20% of total primetime airtime.
03-04 season (18-49 Live only): Top 5 sitcom ratings avg= 6.86. Top 10 sitcom avg = 3.68
09-10 season (18-49 Live +7): Top 5 sitcom ratings avg= 4.12. Top 10 sitcom avg = 3.68
16-17 season (18-49 Live +7): Top 5 sitcom ratings avg= 2.84. Top 10 sitcom avg = 2.39
Television doesn't exist to provide good content, regardless if it is comedy, drama, or anything else. The quality of the content doesn't matter and the "art" definitely doesn't matter. Even ratings don't matter. All that matters is the bottom line.
News magazine shows and reality shows cost almost nothing. Even if the show drops 50 spots in the ratings, a reality show will still make money.
Cost of the TV shows:
Sitcoms - Writers (lots of them), actors (lots of them), sets including lights & sound & editing & stage time & costumes & the set building etc all cost millions before the first show ever airs.
TV News Magazines - On air talent, writers, sets building/design and other behind-the-scenes workers are cut by over 90%. Expenses overall are pennies on the dollar compared to sitcoms before the show ever airs. If it fails, who cares. It cost nothing. If it succeeds, execs make 10x what they would with a successful sitcom.
Reality Shows - See TV News Magazines
To be profitable as a sitcome anymore, it must be a mega hit. It's given a couple of seasons to prove itself or it's dropped. The best writers, performers, crewmembers are turned off by this.
However, premium channels like HBO and now Streaming channels are paid for by the viewers. They don't need high ratings among the "gen pop" masses to be successful. They can broadcast 5 seasons with lower (more exclusive) ratings and still turn a profit. The audience is of the more educated, wealthy, high-brow demographic, so that's where the more intellectual talent is appreciated and finds success.