The cable networks still make them, of course, but why don’t the broadcast networks make them anymore?
Probably (and this is a WAG, just so you know) because it’s cheaper to buy the rights for broadcast than it is to produce an original movie. Also, probably, they found that original made-for-TV movies don’t produce the ratings (and therefore potential ad revenue) that a broadcast of a major cinema production does.
Yeah, but they don’t run a lot of studio movies, either (other than Disney-owned ABC running Disney fare on Saturday nights). Remember when all three networks would run movies on Sunday evening?
Don’t they still make Stephen King movies once in awhile?
Movies are rarely shown on the networks these days, even feature films. Only ABC seems to do it consistently, with occasional Saturday night movies.
Made-for-TV movies were made in a time where movies were big on broadcast, so original movie programming made sense. Now that there’s cable (including the premium channels), you can see movies any time you want. The broadcast networks moved on to other programming.
Made-for-TV movies are a standby for cable, as you mentioned. Certainly, Lifetime and a little thing called High School Musical can attest to that.
By “broadcast” networks, do you mean NBC, CBS, ABC?
If so, I think that particular species of made-for-TV movies is pretty much extinct. However, there is a cable channel that has exciting new films premiering almost every week.
Their latest made-for-TV offering features the talents of John Rhys-Davies*. It’s called Anaconda III. 
*and David Hasselhoff!
The great thing about the TV movies they do on Lifetime is that they at least once upon a time they were all titled something like Breathless Anticipation: Living the Life of a Transvestite Stripper. Then three months later they’d get re-run as Living the Life of a Transvestite Stripper: Breathless Anticipation as though it was a brand new movie.
Reminds me a bit of Frank Thomas and Ollie Johnson’s famous book Disney Animation: The Illusion of Life, which has also been published as The Illusion of Life: Disney Animation.
…and VCRs and DVD players. I think the straight-to-video movie is the modern-day equivalent of the made-for-TV movie.