For one very simple reason. Shows actually have storylines now! In the past pretty much every show, comedy, drama, action, cartoon, whatever followed the same formula. You have your cast of characters, and then episode after episode, week after week, they deal with a particular challenge. Each episode is a self-contained unit with a beginning, middle and end, with no real overlapping story. You'll have references to previous episodes for the hardcore fans, but beyond that, it was all situation of the week that would last indefinitely for years until the ratings couldn't sustain the show anymore.
However, this was limited to North American shows, and is a big reason why I preferred watching anime over american cartoons. My friend actually wrote about this last year and asked the president of NBC (or ABC) why there's a difference. It turns out that doing story-based finite run miniseries is actually cheaper to do than the episodic style of American shows. That doesn't really answer anything, but it is interesting to know.
The funny thing about all of this, is that over the past 5 years or so, with the introduction and massive success of shows like 24 and The Sopranos, televion shows are now following that model and having actual plots that begin and end. If you look at the most popular shows on television now, a lot of them have very distinct storylines, Lost, Heroes and so on. Heck, even reality shows have very clear storylines, it's just that the stories are somewhat unscripted.
For me, shows are definitely getting better. After a very long hiatus of not watching tv, I'm now back to viewing a bunch of great shows very regularly. Thank you HBO.