What ever happened to the half-hour drama?

Now some of you may have differing opinions, but I really like the
half-hour TV drama. There aren’t any new ones being produced that I
know of, but I have enjoyed those that I’ve seen in the past.
I
bring this up because I recently purchased “Doogie Howser, M.D.” and
have been enjoying it again. Now I’m sure there are some of you out
there that hate this series, but I was quite fond of it in my teen
years.
I’m also a fan of Jack Webb’s “Dragnet.” This classic show managed to pack a down-and-dirty plot into a half-hour.
Likewise,
“The Wonder Years” was capable of doing the same. (Incidentally, “WY”
was usually shown in the same hour as “Doogie” when they both aired on
ABC.)
So my question is this: Why aren’t 1/2-hour dramas on TV any more? What’s the problem with the format?
What other half-hour dramas were there?
I could include “Batman:The Animated Series” in this list too, but I’m thinking more along the lines of live action stuff.
For that matter, why aren’t there any half-hour adventure shows? Or sci-fi shows?