Bombs can be rigged to go off without the bomber being nearby. Things like cameras, witnesses, and trained dogs are the only defense there at best, and the potential damage can hit more than one person at a time. It also spreads a far more obvious fear that's harder to control.
The news even with guns reports pipe bombs and incindiaries enough to show that it'll always be a threat. The real danger of it is how impossible it is to stop them from being homemade alongside the potential there is to get away with it undetected, allowing repeat offenses far more than someone spraying gunfire. Even the parts if handled cleverly enough won't resemble anything obvious.
It's why airport security has to be so paranoid.