I'll tell you why. Because the TR forum is full of gigantic, epic, multi-page, book-length TRs that can make you feel tired just trying to make it through the tables of contents. It can be very, very off-putting. Just the idea of writing a short TR in the TR forum can make you feel like you're doing it wrong. So some people just leave a single long post in the Theme Parks forum. Maybe it's time to split the TR forum up into short and long report forums? Or make it more explicit that you don't have to write an encyclopedia just to post in the TR forum? I don't know. I do know I just came back from an awesome, pivotal WDW trip, and for the above reasons the last place I feel comfortable writing about it is in the TR forum.
Furthermore, criticizing people's admittedly misplaced TRs for being too short is mean-spirited and totally not welcoming. It's also ludicrous, considering the whole point of the DIS is to talk about Disney. Not to mention hypocritical to throw in your own "just back" report in a rant telling people to stop posting "just back" reports. Seriously.