Bombs are less likely compared to guns or melee weapons because of two reasons, I figure.
For one, bombs take a lot more effort. No seriously, explosives are harder to get your hands on than a gun and bullets, there's some extensive licensing in place in the US if I'm not mistaken. You can create your own, but that's not as easy as walking into a gun shop and buying a gun and bullets. Some people are able or willing to take that effort, but laziness will keep most would be mass murderers from doing so.
For another, bombs aren't viscerally satisfying. Or at least not as much so as pulling the trigger and seeing the effects of the bullet hitting someone. Given how personally offensive a lot of the targets hit by mass shooters are to the shooter that satisfaction has to be a factor, even if they expect to die in the doing.
For one, bombs take a lot more effort. No seriously, explosives are harder to get your hands on than a gun and bullets, there's some extensive licensing in place in the US if I'm not mistaken. You can create your own, but that's not as easy as walking into a gun shop and buying a gun and bullets. Some people are able or willing to take that effort, but laziness will keep most would be mass murderers from doing so.
For another, bombs aren't viscerally satisfying. Or at least not as much so as pulling the trigger and seeing the effects of the bullet hitting someone. Given how personally offensive a lot of the targets hit by mass shooters are to the shooter that satisfaction has to be a factor, even if they expect to die in the doing.