They need overpower the flight crew and keep them out of the cockpit. This is why the 9/11 attacks included "muscle hijackers" who weren't there to fly the plane. I'd imagine that using a bomb to hijack a plane might go poorly when they realise you're there to fly the plane into a building and not just after money/going to Cuba/other typical hijackings. I think a lot of flight crew might as well just decide to attack you (and risk your "bomb" going off) instead of endangering people on the ground.
Why would the crew realize the person is there to crash the plane? The would be suicide hijacker wouldn't be dumb enough to announce his full plan. He would want to create as much stability and order as he can. Probably would go into the cockpit, lock it and kill the pilots. By the time the people realize what happens, it would be too late.