I'm still trying to figure this one out...
People might praise a documentary, people will say "Oh yeah! I've heard that this one's amazing! You should go see it!", but then the next thing you know, people will buy the DVD because they wanted to see a fiction movie in theaters instead
Can anybody try to explain this to me??
If I understand correctly you're asking why people would rather watch a documentary on DVD rather than at the theatre.
I'll tell you my answer. A documentary is not a theatre experience - there is usually no reason at all to see a documentary on a big screen. But there is reason to watch it on DVD because you can rewind bits that you didn't quite get the first time. And in a documentary it's important that you do understand the bits. In Fog of War or Frost/Nixon every bit of dialogue is important so you don't want to miss it, or if you do miss then you want to rewatch it.
Watching a documentary in the theatre doesn't usually add to the experience whereas for many, many action movies seeing it on the big screen does make a difference.