For one thing, when you're lying down, your feet are generally level with your heart, which makes it easier to circulate blood through your legs. Also, you're going to change position naturally while sleeping in a bed, but you're much less likely to do so when jammed into an airplane seat. Plus most people don't spend more than 8-10 hours in bed at a time, whereas DVT is primarily a big concern on long flights stretching 12-20 hours.