Because it's a poor risk to take - you have this whole product you've designed, and if you built in an unproven pointing device and it flops, the whole product

Because it's a poor risk to take - you have this whole product you've designed, and if you built in an unproven pointing device and it flops, the whole product is a dud. At least with desktops a mouse or trackpad is cheap to replace, so a user might be willing to buy a computer with an unconventional input device knowing that it's only $50 to replace. Buy a laptop and find out you hate the pointing input, you're out 2 grand.