[avar]

As the Wikipedia article you linked to points out 96% of China's population lives east of that line, it would be really odd if most of Apple's suppliers weren't also east of the line.

[ttflee]

If you break these numbers down further to city level, most of them are near the coastline.

[NamTaf]

Naturally; the coastline was the location of major trade ports, and major trade ports are where business grows. It's no coincidence that the Chinese special economic zones all sit around the coast.