(Edit: Am a Swede, and live nearby the Linköping bomb site that hurt 25 this June..)
My uneducated guess as to why the violence is that is has become decentralized. We had a fairly long period with stable power structures in the Swedish underground, there where a few biker gangs that had a few rare and violent bouts bouts but they where infrequent.
Today we have a large number of loosely organized criminals that act independently, fighting over much smaller turfs.
In some sense this might very well be because the old criminal organizations, populated mostly by right leaning criminals, have failed to integrate potential recruits from different ethnic backgrounds.
Sweden has a problem with freedom of speech, PC and political elite, that fail to acknowledge the consequences of their policies.
I'm glad the Serbs don't seem to think it's worth selling them AK-47s and RPGs, yet.
Guns are efficient killing machines but I'd argue they represent a greater chance of being caught when attempting to kill or injure a lot of people at once.
And this is terrorism, per the article, it's just seen as intra gang, not general public. That's also a +1 for a bomb, which can be hidden in plain sight and doesn't have to "work" to be good as a deterrent for rival gangs.