Well its not due to the proprietary connector. Maybe to its location and popularity of the phone, but not simply due to the face it has a proprietary connector. And wait till Apple makes a micro-version of its connector, enjoy buying a $50 adapter for your docks.
Phone makers could just insist on putting their USB / HDMI inputs on the bottom and we could all enjoy standardized docks
Nokia's first party accessory page is here. I bought the Purity earbuds on Amazon
Nokia Lumia 900 Accessories - Nokia - USA
I just buy bluetooth speakers and play music via bluetooth rather than docking my phone