Because the bombs aren't in phone form-factor, they're big vests packed with nails and shit, just set off by phones. People tend to notice when you walk into the middle of a crowd, drop a large bag, then walk away. Now, in situations where they can pull that off, say crowded busses..., I'm sure they employ that tactic.

Also, I imagine there is a heavier emotional impact associated with 'suicide' bombings.

And because the whole point of a suicide bomb is to send yourself to heaven immediately for killing infidels in the name of Allah. Why would you want to carry on living when you could be hanging around in paradise?