[Rover]

Because the mass market is DX ?

Sorry to say but most of the action still happens below the magical 1000$/EUR barrier and FF isn't quite there yet.

About the Tamron - above 1000$ most of the folks will stick to the original manufacturers I'd say.

Why should I buy the Tamron if I can get the Canon 16-35mm f/4 USM L IS for roughly the same price ? I wouldn't.

Because many people are after the magic number - 2.8? Of course the 16-35/4 IS is a better all around choice - after having one for 1 1/2 months I've sold my 16-35/2.8 II without too much heartbreak - but different people have different needs. The Tamron is a more specialized tool due to the lack of filter threads.