Didn't they have one of those flashy "binary liquid" explosives - like the on Die Hard With A Vengence? Those are safe to put in the cargo hold (but not in your hand luggage). I think that's the reason they are targeting hand luggage. If it's in the hold, they can't mix the stuff together at some point during the flight. I guess it could be in a drinks bottle or anything of that sort. Pure speculation though....
Suggested it is nitro based therefore no fuse or detonator needed, just one hard impact and kaboom. The explosion is far far worse if mixed with oxyben bubbles hence the particular interest in fizzy drinks bottles. (Radio 5)