Mobility and space concerns. For most people, a laptop will serve their needs as well as a desktop. Anymore, it's not uncommon to see students bring a laptop to class, or a PDA. Dorms don't have a lot of space, so a laptop provides a good means of having a computer without the bulk that accompanies a desktop.
Yeah I was going to bring my desktop to Uni until I got the money to get my laptop. I can easily put my laptop in my bag, take it to class, type up notes there and then and I've recently registered my laptop for the WiFi in the building so I should be able to access the online resources when in buildings / my lectures / tutorials / classes etc. A desktop just wouldn't allow that much freedom. I have a PDA too but that's used more as a phone than anything else.
I don't know if there's a trend. Desktops still have an advantage as far as ease of upgrading and availability of parts. The vast majority of laptops are limited to RAM and hard drive upgrades. Some can have the CPU and video card upgraded, but that's not the majority. Laptop motherboards don't conform to one specifc standard in terms of size. With desktops, the vast majority conform to the ATX or mATX standard, so it's inexpensive to pick up a new one. Same for video cards. Sure, yours might come with integrated video, but you can go out and pick one up. Laptops with upgradeable video modules are out there, but there's not one central standard. PDA's may start to go the way of the dinosaur in lieu of the smartphone, given how many people have cellphones. Why have a PDA and cellphone if you can have both in one? For the people that are swamped with gadgets, this isn't a bad thing. I don't think we're going to see the desktop go away yet. While, yes, there are some powerful laptops, a comparable desktop is still cheaper not to mention the user-servicable parts are also less. It depends on the needs of the person. Trend, not really. I mean, if there's some reputable data you've got, fine, but I don't see laptops taking over anytime soon.
Yeah I can't see laptops taking over, although for students they do seem much more popular. I'd much rather have my laptop than a desktop curently. Having a laptop means I can take it to lectures and type notes there and then rather than typing them up at a later time. I love my PDA for a few reasons: it has a phone built in, it's from the UK phone company O2 and it's got phone functions - calling, sms etc. At the same time it's a fully functional Windows PDA with a QWERTY keyboard and touch screen which makes using it extremely easy. I can have music, pictures, videos on it and so it's like a media device too.
Factors.... Usability Easibility Quality Budget Flexibility Design Company Reputation Upgradable.... I think most of them were already provided but I've added some as well...