I have to ask myself that same question. Other than what's listed above, I can't think of any other reason. After using different (and concurrently most of the time) sensor formats, I am so used to adjusting myself to see and capture pictures through the viewfinder, regardless of what size the sensor is; Point & Shoots, Micro 4/3 (and 4/3), APS-C 1.5 (Sony), APS-C 1.6x (Canon), APS-C 1.7x (Sigma, yeah I had an SD-9, but it suffered from electrical problems so it was short lived), APS-H 1.3x (Canon 1D, 1D II, 1D III). I am as comfortable with a 50mm lens on a 1.5x crop camera as a 50mm lens on full frame. When I look through the viewfinder, my mind automatically compensates. I can't say I prefer shooting with full frame over APS-C or M4/3, although I have a preference to what lenses go with what camera.
Big Tree - Carl Zeiss Jena Biotar 58mm f2 & NEX-6
Even the M4/3 format sensors have achieved amazing image quality not possible just a few years ago. The Olympus E-M5/E-M1 sensor is as good as most APS-C sensors and may even be better than some 18MP sensors that's been around for more than five years from a certain manufacturer. But the most amazing low light sensor is in the Fujifilm X-E2/X-T1. If you compared the RAW output (on dpreview), it's on par, or even better than the Sony A7 full frame sensor. I am always attracted to weird and unique features and that's why I had a Kodak SLR/c and Sigma SD-9.
For the last few weeks, it has been a struggle for me deciding which camera to upgrade to. The Sony A7 is my top of my list with great image quality and using all the legacy lenses as they were designed is an alluring feature. Then there is the Olympus E-M5/E-M1 and the Fujifilm XE-2/X-T1. These two cameras keep distracting me from firmly committing myself to the A7.
The E-M5/E-M1's In-Body-Stabilization is something I have always wanted, and I want to replace my dying Panasonic G1 so that I can start using those wonky c-mount lenses again. The Fujifilm X-E2/X-T1 for its amazing image quality, especially high ISO. Besides, the sexy look of the X-T1 gives me wet dreams; I can not shake its image off my mind! This is a camera with retro done right.
So, full frame digital is not the ultimate and the end all of camera quest. To some, unique features like in-body-stabilization is more important than full frame. If I get any camera other than the A7 in the near future, it also means full frame is not as important to me as other camera features.