The current owners have sure been trying to turn it into one with half-assed "features" like open ended questions and the thankfully killed off (for now) beta redesign.
A forum records free-form conversations between specific people, where every post can be edited at most by its original author (if even that). SE sites are libraries of questions that may be generalized beyond the original author's circumstances, and if someone's post does not fit the rigid Q&A form, it may be moderated away.