I actually *do* think that vertex-colouring is more interesting than edge-colouring, although I fully admit that this is my own personal bias.  Instead of focusing on why edge-colouring is uninteresting, let me highlight why I think vertex-colouring is interesting.

**Connections to topology**.  Along these lines there is of course the [4-colour theorem](http://en.wikipedia.org/wiki/Heawood_conjecture), [Heawood's map conjecture](http://en.wikipedia.org/wiki/Heawood_conjecture), and [Grötzsch's theorem](http://portal.acm.org/citation.cfm?id=794187). For planar graphs there is also a suitable notion of duality of colourings via [flow-colouring duality](http://books.google.ca/books?id=aR2TMYQr2CMC&pg=PA152&lpg=PA152&dq=Flow+colouring+duality&source=bl&ots=0xZVPNk7ze&sig=uQ5BkjJjjUCxiSUBV7ZPORdz8jk&hl=en&ei=PxGZS8DiEYmVtgeK-fGwCQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAYQ6AEwAA#v=onepage&q=Flow%20colouring%20duality&f=false).  Kristal's answer is an instance of flow-colouring duality in action.  That is, it suffices to prove the 4-colour theorem for planar triangulations.  The dual of a planar triangulation is a cubic planar graph.  So, it suffices to show that cubic planar graphs have 4-flows, and it's sort of an accident that 4-flows for cubic graphs actually correspond to 3-edge colourings.  

**Structural graph theory**.  Perhaps the most famous open problem in graph theory is [Hadwiger's Conjecture](http://en.wikipedia.org/wiki/Hadwiger_conjecture_(graph_theory)), which connects vertex colouring to clique-minors.  So, high chromatic number can actually force some structure, while high edge-chromatic number just forces high maximum degree.