Because the useful volume of a laptop is somewhat limited and in the cabin you can ensure placement against the skin of the aircraft. In the baggage hold it will have been scanned better and by a process that is less susceptible to compromise by a single person, it will have been placed in a somewhat random position inside another container with a fragmentation blanket around it, etc. Yes, you can certainly bring down an aircraft by putting a bomb in luggage, but a lot of effort has been put in place to prevent this exact thing.