This issue is far more complicated than one cause. For starters there's what I call the "Ghetto Effect" and the "Hollywood Effect". There's also a simple matter of resources - no media outlet can possibly devote significant time to each and every case.
By "Ghetto Effect" I mean we tend to have an assumption about the ghetto - namely that bad stuff is supposed to happen there. So if one hears about a shooting at a school in the hood, or a gang rape, or some other crime, the general public doesn't blink. However, when the same thing happens in a well-to-do neighborhood or to a celebrity, all of the sudden there's panic. We say "how can such a thing happen here?".
Also, there's a rather well known rule within certain neighborhoods - don't snitch. People won't talk to the police, so what good is highlighting the crime on national TV? In well-heeled neighborhoods, people are more than willing to talk, even offer a theory. So, guess where the mass-media resources go? Right - to majority white and middle/upper class cases.
The "Hollywood Effect" is two-fold. One, mass media has to tell a compelling story to draw people in, just like Hollywood has to create an interesting story to sell movies and TV shows. In fact, today the news is often in direct competition with Hollywood. As unfortunate as it is, one can get real numb to these stories after a while. They all start sounding the same with different characters. So even if there were a dedicated program to spotlight all 800K kids each year, very few people will watch. And consider this - there are 525,600 minutes in one year. So for one channel to offer round-the-clock coverage, each kid would get an incredible 39 seconds on the air, assuming no commercials or any other breaks.
Two, the Hollywood effect also helps create distance. Just like in the Ghetto Effect where people accept such crimes as long as they are "over there", Hollywood often dramatizes those same kinds of crimes in various TV shows and movies and audiences gobble them up, knowing that it is fictional and thus "won't happen to them".
On top of all of that, there's the news cycle to contend with. Anyone unlucky enough to go missing on hyper-active news days - like 9/11/2001 - are simply out of luck. News organizations have their hands full when something like a catastrophe happens. When the media is in need of a high-profile case, then missing persons becomes a hot item. When the need for such stories falls, so does the coverage.
Finally, this is why I do not believe this issue is race-related. It is true that an overwhelming number of national stories involve white females and it is also true that minority victims are 20% or more of the overall total of missing kids, yet that still leaves a substantial number of kids who do NOT raise national attention.
Since all missing kids cannot possibly be featured on national programs, it is VITALLY important that they are at least acknowledged by local news. Fortunately that is the case, but even local news cannot put every missing child on the air.