You probably already know, but just to be sure: the 640 XL is far from a high-end phone.
I'm not aware of MS ever officially stating why they aren't servicing the high-end market. I think it's because pretty much every high-end WP device to date has failed. The only WP devices that have sold in meaningful numbers were the lowest end devices, like the 520 and 535. I suspect MS has weighed the costs of releasing yet another loss-leading high-end WP device, against the costs of not doing so and losing a good portion of the high-end WP fan base as a result. I suspect the former turned out to be a lot costlier than the later.
That won't change until MS can release a high-end device that is more likely to sell in larger numbers than they typically do. That will require more than the typical annual refresh with the incremental bump to specs.
I wouldn't say the 640 XL is 'far away' from a high end device. I'd consider it a midrange and considering it's various specs it outperforms certain high end phones in some key areas. Screen and processor aside, if this was an iPhone with similar specs, it'd be considered a high end. But I digress.
It would help Microsoft if they fought to stop releasing carrier variants that have slightly better features than the previous one(928 vs 920 or 925) and if they marketed their own products and offered incentives for their products. The Office 365 with the 640s(and the 735 I think) is a great one.
Offer Xbox live on these phones for a year.
Etc etc.
A large part of the reasons these phones aren't desired(I know as I work to sell these phones) is simply because nobody knows they exist. Partly because of carriers, partly because of OEMs(See Samsung and HTC's poor attempts)...actually this is a larger part tbh and partly because Microsoft barely acknowledges windows phone. You can't sell a product if people don't know they exist or what they do.
The sad thing is Microsoft poor more effort in Kin's marketing in the US than they ever did for Windows Phone. And that's saying something since Kin died in less than 3 months(Thanks Verizon).
If what you want is a 640XL from AT&T, buy one of those unlocked, unbranded, up-to-date, dual sim Chinese ones you can get right now with better specs than the crippled version AT&T will eventually release. Set it up, put your AT&T sim in it. Walk into the AT&T store, show it to them, and let them know you got tired of waiting. Upload the video here so we can laugh at them. Why be tied to a carrier when good Lumias are so inexpensive?
Couldn't have said it any better myself. It's incredibly frustrating when they cripple the devices then wonder why they had less than expected sales and then try to blame it on the manufacturer.
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*Raises hand*
640 XL owner here in convict orange color

Unlocked and from Hong Kong.
I am happy to have my first factory unlocked phone free from Carrier updates.
What does bug the hell out of me is the fact that that not only do carriers cripple these devices(although Verizon did good in bumping the 735 storage up, so kudos to them) is Microsoft handicaps them.
With phones like the 635 there is no real reason it didn't have a FFC. Look at the 530..no FFC. But the 435 gets it?
There's no reason why the 640 has nearly the same specs as the 535...except it got a boost in the back for 8 megapixels but the camera on the front is 0.9 megapixels.....but isn't there a 540(which is the 535 I believe with LTE?) that has the same specs as the 640 except it gets the camera in the front.
There is absolutely no reason the 830 has a weak camera(especially for its high price) in the front...when you released other pureview cameras with at least 1.2 megapixels.
Why cripple your product and then customers get cheated when you release a slightly better product a few months later? Which is precisely a situation Samsung/HTC used to do in the early days of android. The original Galaxy S and S2 phones were incredibly different depending on which carrier you went to and had(It wasn't until Galaxy S3 that they begin to share a common design and feature set).
The only phone ATT has gimped, to my knowledge is the 1520...why idk....but most of their Windows Phone usually are not gimped. They usually get the original versions of the phone and no variants(except the 925).