It's because they don't sell well.
Trek debuted the Portland in 2006 as the first production road bike with disc brakes. They discontinued it due to poor sales after the 2011 model year.
I don't have many pics of the non-drive side of mine (a 2006 model). Here's the only one I could find fairly quickly.
Other than four-seasons all-weather commuters who are willing to drop some serious coin on a bike to ride in the rain and snow, there's no compelling need for disc brakes on a road bike.
Part of it too is that dealers couldn't figure out how to sell them. I own the only one sold in my area. There are six Trek dealers just on my side of town. Five of them have never even seen one in real life, until I ride in the door, then we're instant celebrities. They all think it's cool, but they don't have a clue as to who they'd sell one to.