I think sales volume could be a factor. If there are only a few hundred potential customers in the world for your product then you can't benefit from economies of scale. As Arduino has become so popular, and even a lot of people who don't use the Arduino IDE or the Arduino libraries, buying these boards simply as a microcontroller breakout, large manufacturing runs have become feasible. Back in the days where it was just a small niche hobby market that was not possible and these Chinese manufacturers wouldn't consider making them.