First off, I love Jeremiah... I enjoyed the first season immensely and am really excited for the second season. Before it came out, I was having a love / hate relationship: I wanted to love it because it was JMS (which ended up happening), but wanted to hate it because it was Luke Perry (who I now like - give him good dialogue and writing, and while he's no Orsen Welles, he's good enough).
About 6 or 7 years ago, HBO entered a realm no cable channel had before by offering a series... "Oz". It was jarring, like nothing you've seen on TV. It was never a smash hit, however, still a very interesting show, if you can stomach the gore. Not to mention, if you're a female looking for a show with male nudity instead of female nudity... it is your Mecca!
HBO had dabbled with "The Hitchhiker" and things of that nature in the past, but "Oz" was its first real try at a series. However, until "The Sopranos" got huge by breaking all the drama rules, and "Sex & the City" got huge, by breaking all the sit-com rules, no cable shows were hits.
Showtime has only recently experimented with original series. They're seeing marginal success, but this process takes time. I don't know the figures - Joe D. probably does! - but there are still HUGE parts of the US (not to mantion internationally) that don't get to see these programs.
In my opinion, "reality" TV has also dealt the cable channels' original series a blow. Now that network stations realize they can satisify the American masses with the equalivalent of a neighbor acting stupid, there is no reason to spend money on network dramas. This, in turn, has caused the great majority of American masses to turn off network dramas, and in turn, cable dramas.
Again, in my opinion, this has caused much less "buzz" about even the most popular cable series in America, such as "The Sopranos." When Season 4 was being filmed, there was an article practically every day in one of the entertainment rags. Now, you can't find anything even on the HBO shows, let alone the Showtime ones.
The long and short is that the Showtime originals don't have the reputation of the HBO originals. Critics will give any new HBO series a good review, because its done by HBO... "it must be good." Showtime doesn't have that reputation... yet.
UK comedy is also making a comeback through all of this... "Coupling" is one of the *funniest* shows EVER.