More reasons

Runs QtMoko, SHR, Debian - fully supported
Made in Germany under proper working conditions
fully documented schematics
fully open bootloader, kernel - no secrets, no overprotection
replaceable battery
3D CAD data available to design and 3D print your own case
FM receiver/transmitter option with RDS (that is rare)
complete set of sensors (barometer, compass, accelerometer, gyroscope, GPS)
B2B connector to use it as a System on Module
very high display resolution (282 ppi) [http://en.wikipedia.org/wiki/List_of_displays_by_pixel_density]
no online accounts and registration required
offline gps navigation and dictionaries
all applications are free and open source
fast processor and big RAM is not needed - we have fast non bloated software
long term support - freerunner is after 4 years still actively supported phone
gta04 kernel is exceptionally good - most of drivers are in mainline kernel and gta04 kernels are updated with every linux release
separation between modem processor and application processor keeps your application software in control of what the modem is doing (or not doing)
... and a friendly community allowing for direct discussions with the core developers

GTA04