This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
"Blue" is just one of several high-profile series ("ER" and
"Ellen" are two of the others) that will be taking an extended
vacation in March and April to provide opportunities for
networks to showcase promising mid-season replacements.
There are two reasons behind ABC's decision to do this. First,
there are only 22 episodes in a season of NYPD Blue, and,
because of the all-important "sweeps" months in November,
February, and May, 12 of those episodes have to be committed
to air during those months. That leaves only 10 other episodes
for the other five months. So, during March and April, if ABC
aired Blue, there would only be one or two new episodes.
And, because of the sheer volume of successful dramas and
newsmagazines in the 10 o'clock hour on network television
these days, there are very few timeslots available to test out
a new show. So ABC has chosen to put "The Practice," a new
legal drama from David E. Kelley ("Picket Fences," "Chicago
Hope") on in the successful "Blue" slot, rather than risk it
in a worse timeslot (like, say, the Thursday at 9 slot that's
killing "Murder One").
Complaining to ABC won't do any good. Their minds are made up
on this. And unless you have a Nielsen box, you really can't
vote with your remote, either.
But if this upsets you, keep in mind that "Blue" is a big
success because it got a lot of publicity about the adult
content when it launched (and was good enough to keep all
those curious viewers week after week). Not every show is as
lucky. At one time or another, nearly every one of us has
started watching a show that got cancelled prematurely due to
low ratings. Maybe some of those shows would have attracted
more viewers if they'd been given a chance to air in a better
timeslot for a few weeks.