Probably because the unit only shipped with 1-2GB of RAM, and was limited to a maximum of 4GB (I don't know this for sure, just speculating). So there'd really be no benefit to a 64-bit OS, and it'd just confuse people who buy some off-brand MP3 player or something, there are no 64-bit drivers, and then people complain that it won't work with their computer. The computer vendors are ALWAYS the ones who get the blame in these situations. I've worked for a major component vendor, and believe me, it's quite impressive the lengths people will go to in order to blame the vendor. Maybe you're not that sort, and if so I commend you, but the fact remains there are several people out there like that. One example that comes to mind is some guy basically threatening to never buy any additional products from the company I worked for if I didn't give him an upgrade to a newer model of the component. I told him there was no way I could justify doing that, because at the time he hadn't even submitted an initial RMA. So naturally when he tried to go tell the world what happened, he made it out like I was trying to deny him an RMA. Sadly I had to rise above posting the emails the guy sent me where he's acting like a jerk (if I'm generous). There was another case where someone sent in a component, there was physical damage to it which couldn't be repaired... The guy was offered two discounted rates for a replacement board, refused both of them, then wanted the company to send the part back to him via overnight FedEx on their dime. When you heard him tell the tale, somehow these little details got left out. I'm sure others around here who've worked a customer service type job could chip in with a seemingly endless sea of similar stories.
So say you're Dell in this instance. You load a 64-bit OS on there, and now you have to pay to have your support call centers tied up with people calling to complain about this sort of thing. We can complain about how most call center reps are poorly trained at some other point, for now it's basically that people with legitimate support issues are now stuck waiting even longer, and Dell is picking up the tab for the long distance charges, which will ultimately be reflected in the price of new products. While it would be wrong to act on it, we should all have the impulse to beat these people about the head and ears, because their selfishness is ultimately driving up the cost of goods for the rest of us.