For one, XP x64 was never intended for mass consumption. There's a reason it was only sold as an OEM release, and never promoted in any way. It was really only ever intended for people who do a lot of things that would benefit from having a 64-bit OS. But of course some people found out about it, thought it'd be really cool if they had it on their home system, so they rush in without thinking, and oh look, it goes badly for them. Vista was always meant to be the first real 64-bit release, Microsoft just cow towed to pressure from some of its deep pocket customers and whipped out XP x64 since Vista was still stuck in that quicksand bog.
Vista x64 OTOH is being actively promoted, it was released right along side the 32-bit version, unlike 3-4 years in with XP x64, and unlike the last go around, it's becoming more and more common now to find systems with 4GB+ of RAM. Best Buy, for example, is selling a number of Dell brand models with 4-6GB of RAM, and the person who started this whole discussion found a laptop shipping with 4GB of RAM that also came with Vista x64.
So, I'd say the big difference this time around, is that with XP x64, it was never intended for Joe Q. Public, but Vista x64 is. It was released, and has been maintained, along side it's little brother in the 32-bit version. Hardware vendors are just being predictably cheap, and procrastinating on coming up with 64-bit drivers. I've read enough of your posts, Bob, to know that you know as well as I do, there's no technical barrier when it comes to making 64-bit drivers. In a lot of cases, all that's probably really required is a simple find and replace in the source editor to change a few API calls and then to recompile the thing as a 64-bit binary. Vista x64 just tends to insist on signed drivers, which means a couple hundred dollar fee to Microsoft's little racket. So when you're selling some cheap inkjet printer, almost certainly at a loss and hoping to make it up with hugely inflated ink cartridge refills, you need to sell quite a few more cartridges to just break even if you have to pay off Microsoft. So hardware makers aren't going to do that until people start complaining loudly about the lack of 64-bit drivers. They're going to try and avoid it for as long as possible to try and milk things as long as possible.
In the business world, pretty much everything comes down to money. And unfortunately, with the pressures put on managers from shareholders who only care about making a few extra pennies per share for themselves, long term profitability is almost always compromised in order to meet short term quarterly projections. If HP, for example, had started developing 64-bit drivers for all of their currently supported printers the day Vista was released, over the long term they probably would have saved money, and be selling more units. That won't ever happen, because if some manager fails to meet quarterly projections, they are either out some bonus pay or possibly a job.