Fair questions, and I don't think there are many good answers.
Short answer: I agree. There are a lot of worthwhile charities and missions, and you can't do everything. You've got to pick one, and it's easiest to pick one that matters to you somehow. The media helps to raise awareness of those things they choose, like Madeleine McCann, to the detriment of other things. So in order to get the full picture of things, you have to look to multiple sources of info.... news from places other than the major ones.
A couple of facts, though... breast and prostate cancer are the most common cancers in the world. True, there are no fundraisers for prostate cancer (that I know of), but as men get older, they get a fair amount of education about PSA, the substance they check for to determine if you've got it. There's also the whole finger/rubber glove thing to check for it every year...
Lung cancer is deadlier, and every pack of smokes comes with a warning (that's ignored

). But it's not as common... apparently, enough people are getting the message that it's not as prevalent.
Why breast cancer and not MS? No good answers. There are MS fundraisers... a quick google found bike races, walks, and other events. I think part of it comes down to marketing, pure and simple. The breast cancer folks have some good people making sure the word gets out about their events, and they have done well. They got some celebrities to adopt the pink ribbon thing, and it became an easy way to demonstrate affinity. And some people probably wear the pink ribbon just to fit in...
My wife often asks about the promotion of specific cases, like Madeleine McCann or the missing kid case down in the southern US now. Why them, and not someone else? Why the cute white kid instead of a cute black kid, or hispanic kid, or some other kid? Why Lacey Peterson instead of the thousands of people who disappear?
I think the media chooses ones they think will sell, and we're stuck with them. And that ain't right