Two main reasons is that they could potentially interfere with sensitive equipment in the cockpit (although very unlikely), and that they want your full and undivided attention during take off and landing in case they run into any problems and need your attention. It's impossible to tell people to assume the crash position if they've got headphones on, and they don't want people worrying about their laptops during the extremely unlikely event they have to make an emergency landing. Problems - rare as they are - tend to occur during take off and landings so they want to take the extra precautions then.
