Hard drives!

[lolsteve]

Steve main reason is power consumption. Remember with a laptop the designers have to think a lot about power consumption where as with desktops, most couldn't give a toss.
SSD also has a greatly reduced power consumption meaning you may see a increase in battery time as well as access times.

Ah good point I didn't consider power consumption, I always assumed due to the HDD being serveral spinning disks they'd be spinning at a point where performance wasn't sacrifaced too much but at a less dangerous speed. Since if you shake a laptop hard enough it'll crash