It's hardly surprising there's so little difference between WP phones, the spec is tightly locked down by Microsoft. Specifically the software can have almost no customisation.
While I actually quite like the idea of near stock devices, phone makers put more into distinguishing their brand and individual devices by software than hardware spec (which is now less tightly specified by MS). Android sales show the public disagree with me, voting for variety over conformity. Meanwhile the carriers selling most devices simply can't resist messing with (and messing up) the software.
It's should surprise no-one that manufacturers and carriers aren't keen on a device so locked into Microsoft branding. Unlike iPhone they can afford to boycott it, to make half hearted designs to test the water (and fend off license blackmail over Android).
Samsung don't need these sales and the mainly "big screen+heavy skinning" strategy that got them to #1 in smartphone is a poor fit for WP license restrictions. HTC and Nokia are fighting for survival, they need to try harder - and it's winning them a bigger slice of a microscopic pie.