Why should they? For most people a "smartphone" is synonymous with Android or iOS, and the customer has a pretty good idea of what they want when they go into the phone store. Android manufacturers have the same problem with people that "want an iPhone". A salesperson that comes along and says "Hey, the Nokia Lumia will probably suit you better" will appear pushy and lose the sale, so what benefit is it to them to try and force WP7 on people who already know what they want?
I think that the carriers bring up valid points here. Windows Phone is barely advertised here in the UK (when compared to the iPhone and Android), and Microsoft being the minority share in the mobile space can't simply expect to switch to WP7 if they haven't heard of it. Nokia's marketing is OK, but to me the Lumia adverts advertise it as a feature phone rather than a smart phone. At the end of the day, Microsoft need to spend out the ass to market WP7, they're the minority here, and Android and iOS basically sell themselves because of their popularity. This isn't to say that WP is a bad OS, far from it, it's a great OS, but marketing is everything, and Microsoft isn't putting enough effort in over here in the EU.
It's worth remembering that here on Neowin, WP7 is well known, and as popular as iOS and Android, but this is a very pro-Microsoft site, and this is to be expected. In the real world, the number of WP7 phone owners in my circle of friends recently dropped from 2 down to 1 as one of them opted for a Galaxy Nexus. Nobody has heard of Windows Phone, and many of those that have are still butthurt about Windows Mobile.