From what I can find, about 3 million Nexus 7s have been sold. So comparatively, the market for iStuff accessories is HUGE. On top of that, because Android devices don't have a standard for docks, accessories are only relevant as long as the device they are specifically for is relevant. For tablets, that's what... 9 months? A year? Meanwhile iStuff docks have changed only once since launch that I'm aware of, and you can still use a lot of older accessories with an adapter.
Apple, meanwhile, has sold a total of 85 million iPhones in the U.S. since 2007 launch and 34 million iPads since 2010.