This is a more complex question than one might realize. The modern political world (since the early 20th century) has been marked by a dramatic increase in the power and efficiency of national military and police forces, both in terms of military hardware and surveillance capabilities. This has pushed counter-national movements to organize as partisan forces: loose, decentralized, and lightly organized groups and individuals working for the common purpose of opposing organized regimes. We have a lot of terms for that these days: terrorist cells, lone-wolf actors, radicalized individuals, etc., but the common theme is that any movement of this sort is going to have a wide, diffuse, largely non-coordinated assortment of individuals who follow the movement's lead and principles while having only minimal contact with the movement itself.
ISIS was unique in that it had the ambition to establish a new state of its own (not merely terrorizing, subverting, or taking over some established state), but it still had a far-flung net of partisans who largely agreed with its intention without any effective connection with the more organized core of people working militarily in the Middle East. It's an interesting but difficult question whether the actions of these partisans can properly be attributed to ISIS; certainly they can be attributed to the broad movement, but not necessarily to those groups busy trying to establish a Caliphate through normal military means.
As a general rule, only the weakest and most disempowered groups undertake terrorism, because striking at 'soft' targets from hiding is the only way they can exert influence. The more power a group obtains, the more inclined they are to organize and assert their power directly, the more concerned they become with reputation and legitimacy, and the less they want to be associated with mindless, non-directed violence. ISIS at its height was an effective military force with pretensions of being a full-fledged state; terrorism would not have been in its best interests. But partisan actors are notoriously difficult to control since they act from ideology, not pragmatic concerns.