I think a lot of it has to do with the number of AR's out there, the number of rounds the average AR owner runs though them, and the lack of factory ammo out there.
There are a huge number of AR's out there right now. People have been buying them like crazy. When you have hundreds of thousands of any particular rifle out there, you are going to see more scary KB video's just by nature of the numbers out there. For example, how many Lamborgini's do you see on the side of the road broken down vs Chevy's? It's not because Lamborgini's are more reliable, it's because there just aren't as many to break down.
Then you have to consider the number of rounds the average owner puts through their rifles. The average centerfire bolt action shooter fires maybe 50 rounds a year (and I'd put it at even less) compared with many, many more from most AR owners. Again, a numbers game.
When people can't get the inexpensive factory ammo, they are more likely to consider buying that big plastic bag of ammo at the gunshow of unknown origin.
One last reason, AR's are more likely to actually get filmed at the range. Partly due to the average age of AR owners being lower than the average age of other firearms (from my personal observations). Partly due to the fact that some yahoo rapidfiring his new AR is more exciting to watch (or so the people taping think) than some guy shooting a round every 30 seconds with a bolt gun at a target the camera can't even see.