^^^
I take your point, and they do target cases etc towards the popular models like the Galaxy series. But even when you talk about Samsung having a high market share, that's across a range of products all with different designs.
As far as cases go: One design for the 3G and 3GS, one for the 4 and 4S, one for the 5. As a manufacturer you've designed hardly any cases and always hit the biggest selling single model.
The case is even more compelling for speaker docks. Until the iPhone 5, the iPhone series retained the same connector and basic shape, not to mention outputting audio through that same connector. As a manufacturer, you could create a single device that was suitable for most iPhones. Not quite so easy with Android. Even now that phones are all micro-USB, they're not in the same place on each model. And you need Bluetooth or a 3.5mm jack for audio.
I'm not trying to run Android down here at all. And there are a good number of cases and accessories available for the top selling models. But I can see why, from a manufacturer's point of view, that it makes far more sense to make stuff for the iPhone.