I used to watch The Simpsons religiously. Meeting with friends before class in middle school and even high school unfailingly meant an exchange over last night's episode (re-runs weekdays at 6:30pm in addition to the new Sunday material). Around the turn of the century the quality dropped precipitously, and by 2001 I stopped watching entirely. Some potential reasons for the decline I've heard bantered about: the original writers have all departed, Matt Groening had burnt out/moved on to other projects of limited success like Futurama, the show and its cast had run its course, episodes increasingly focused on abrupt antics rather than characters, animated comedy peaked in the nineties. These all probably play a part.
I'd also offer that comedies have to be politically balanced to retain a large audience over a sustained period of time. While the show has always leaned left (Lisa, the precocious ethicist and the show's most thoughful character is also the show's marquee leftist (although Mayor Quimby, a Kennedy Democrat, is deplorable) while Republicans are portrayed as fitting three different molds, none of them very flattering: The erudite and intelligent but evil, money grubbing played by Mr. Burns, the religiously dogmatic and uncritical played by Ned Flanders, and the Limbaugh-listening male WASP buffoon Homer), it used to give conservative themes a legitimate hearing and take its share of potshots at the left. In Lisa the Tree Hugger, while her cause is shown to be noble, the radical environmentalist Jesse Grass is an egocentric maniac believing himself to have special powers to control nature. In Lisa the Vegetarian (my favorite episode), one take home message is that the imposition of one's beliefs on others cannot work in a free society, no matter how noble those beliefs might seem to their propagator. In Radio Bart, when the putative heroism of hapless victims is directly questioned by Lisa, Homer angrily responds that of course the boy who fell down the well is a hero because, damnit, he got trapped in a well! I could go on for hours (and would be happy to if the show strikes the interest of a reader via email or in comments) but many people would/already have tune(d) out.
Since 2000, I've seen a few episodes. One had to do with Homer and Marge trying to escape the children that have ruined their love life together, another that was so explicitly antithetical to the Bush administration's war on terrorism tactics that I could scarcely believe it was the same show, and a third where Fox News was portrayed as an RNC wing while no other media networks were touched. It's not that I'm bothered by it--on the contrary, I largely agree, but obviously that's not what I go to an animated comedy for. It is annoying to see blatant partisanship, not to mention unentertaining. Given the relative sophistication of the show's fanbase, a huge dropoff in social critiquing is easily going to be detected and resented. If the purpose is to allow us to examine our contemporary world through the lens of one that is just a finger far enough away to allow us to suspend our disbelief without losing our real world perspective, leaving half of reality unscathed makes for half the show. Some politically sensitive fans will abandon such a series, and people well versed in current events will, irrespective of their political leanings, be put off by such tactless partisanship.
Not surprisingly, the show that started a seminal first season with 13.4 million viewers seventeen years ago boasts only 9.4 million today.