The explanation I have heard is that because laptops are so "dense" electronically, they are difficult to see through on the x-ray. With multi-layer circuit boards, hard drives, etc. it might "hide" something. By removing them from a bag it eliminates having to x-ray through the power supply, mouse, network cable, etc.

Now if it were a laptop bomb in checked baggage instead, not sure how you would be safer.

Check the Travel Safety and Secuity forum and you will probably find more on this topic.