At least I prefer to work on supporting interesting hardware which RPi 3 is clearly not. For me personally the various Raspberries have one single feature: That's the ability to use HW accelerated video encoding (identical on all RPi models since the job is done on the VideoCore IV and not the ARM cores) but fortunately we can now also use Allwinner boards (see github repos of community member @lex and various threads in H3 and free forum).
From a user's point of view the only great thing about Rasperries is the huge community, this advantage would be lost by switching to Armbian. So supporting these overpriced pieces of hardware really makes not that much sense IMO
The proprietary boot process and the ARM cores not being first class citizens (the CPU cores do not even know at which frequency they're clocked and information like /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq is cheating on you) also make working with this platform less fun. Also Armbian is a lot about pushing the envelope (optimize settings, tweak kernel and u-boot) which is simply not possible on Raspberries since a lot of this stuff happens inside the properietary so called 'Firmware'. Armbian would just be another lame rootfs on these devices.
BTW: For my use cases I/O and network bandwidth is somewhat important and here even the cheapest devices Armbian currently supports (NanoPi NEO and soon Orange Pi Zero) easily outperform RPi 3.
