The Raspberry Pi was a new system with weak CPU performance (~300MHz Pentium II single core performance) and now has thousands of developers, good "official" compiler support and about 30 operating systems have been developed for it. Cheap hardware attracts users and users attract developers. Learning new APIs and hardware doesn't seem to be a problem when there is plentiful cheap and standard hardware.
You forgot a few important points:
* open hardware, free documentation
* no need for proprietary drivers
* no company that tries to dictate what you run on it
* no legacy demands
* tiny, low power consumption
And the rpi is just one of many and nothing new or special. What the RPi really brought was lower price. And a great expansion port. I used to have a few gumstix and various other boards, but they were all way more expensive. As for APIs, nothing was new with the RPi, Linux and BSD on ARM is old news.