Most arduino projects only use a few I/O pins and very little processing power. Many hobby projects could be made with a much weaker pic processor, and many could get by on the basic 8 pin pics. Many people don't know that the simpler solutions exist, because they only see arduino stuff all over the web. The full development board is way overkill.
Additionally, with current arduino setups, it is fairly simple to make a clone around an ATmega chip. All parts are soldered easily through hole, and the schematic is easy. With a 32 bit surface mount chip, the schematic gets complex enough that most hobbyists are now scared off by the hard soldering and the crazy layouts. The open source, easy to clone nature of Arduino that made it what it is today is incompatible with the new high-end boards, and people will have to pay more for the official dev boards, or something else professionally fabbed.