The reason for this is simple: you're shooting digital.
It's hard to get a nice rich brown. Even on a computer monitor, its nigh-on-impossible. It's the same with Purples and Blues as well as certain shades of green. The colour spaces used for screens (RGB) just doesnt allow them.
Photo.net has a good diagram, produced here:
The solid line shows the colours you can get from the 16 million using the RGB space (dotted is CMYK, used for printing). Your lovely browns are off to the right of the triangle where the red and orange meet. Purples are down below the triangle, true blues are as well, and greens are all off the top. As you can see, the CMYK space is even smaller, meaning anything you print will have even less variation! Keep in mind, as well, that the colours show here are at 100% luminosity, so brown would be a darker orange.
This is just an advent of digital: we learn to live with it.