They don't. Media companies today aren't that interested in very narrow topics for their channels. They want as broad a base as possible to attract many different people, not only those who are interested in, in this case, weather forecasts. More people = more advertising money.
Perhaps sports remain the only major exception. Just think... TLC used to show educational programming, and now it's Honey Boo Boo and pregnant teenagers. A&E used to be literally Arts and Entertainment. Bravo - same thing.
But look what happened with G4 for example: when it first went on the air in 2002, it was originally a video game and technology-themed channel. After when G4 bought out TechTV in 2004, Comcast (the owners of G4) wanted to reformat as a reality-themed channel. Obviously, that concept flopped. Same thing happened with Fox Reality Channel, it too also failed in the ratings. It seems like every cable network wants to show reality television these days.
But of course. So-called "reality TV" is usually among the cheapest programming that can be made, and, since it's still the "hot thing" out there, it generally attracts the most viewers from the widest possible audiences. There's no question that a large amount of programming has little to offer in terms of culture, but it does
entertain - by its very nature there's suspense built right into the program from start to finish, whether it's a competition or a big rig driver trying to cross a snow-covered highway. And did I mention it's cheap? No script writers, the "stars" are paid less than big Hollywood union actors, and so on.
In the case of The Weather Channel, it could continue to attract a small but loyal following of viewers who tune in periodically for weather information, plus an even smaller but
very loyal following who watch the channel religiously, but that would still alienate the majority of the viewing population who would tune to the channel either very briefly and irregularly or not at all. Or... it could fill the schedule with reality TV to attract a much wider audience that isn't interested in weather forecasts, but wants to watch reality programming... as if they don't watch already.
NBC, like the rest of the commercial media in the United States, isn't in business to "educate, inform, and entertain." It's in business to make money, and it makes money by selling advertising time. When you think about it, the commercials are the real reason for television, and any programming around it, which just might be "educational, informational, and entertaining," is just filler to make sure people watch the commercials. That's why there's always so much uproar whenever some new piece of technology comes out to allow a viewer to bypass commercials. First VCRs, then recordable DVDs, then DVRs... the more ways people have to
not watch commercials, the less value advertisers think they get for their money, and the more worried the broadcasters get that they're not going to earn as much money as they have been previously. (Many already aren't.)
I already mentioned how TLC used to be The Learning Channel. There isn't a chance in heck they'd go back to showing educational documentaries and classroom-type programming. Honey Boo Boo, pregnant teens, and couples rushing all over the place to get married make the network a
heck of a lot more money than ad-free Cable in the Classroom programming and documentaries about the various kinds of rocks one finds underground, for example. TLC has abandoned its roots so thoroughly I doubt too many people younger than I am even know what TLC originally stood for. The Weather Channel seems to be veering ever closer to that same edge. The day will come when it either just stops showing weather programming altogether and either changes its name to TWC, or The Weather Channel becomes a channel showing weather-themed reality shows, but no forecasts. And then TWC will find its own Honey Boo Boo and be like every other network out there.