Obvious analogies for both of them:
4chan/2ch (Imageboard)
vBulletin Forums (Forum)
Why aren’t imageboards a popular choice among those who wanted to host an electronic bulletin board system?
Forums are older. Images are big in size which took ages to load back in the old days (5+ years ago) not to mention you had to have this huge block of heavy plastic with lenses and shit that took pictures for you and it took like months to upload them to your computer.
And imageboards are relatively useless compared to forums who inhabit vasts amount of knowledge.
Imageboards are the cesspits of the internet. Spend no time there. If you must totally waste your time, come and work for me, for nothing, making the tea.
Cas 
I do know that imageboards can act like forums, in which uploading images are optional. So, why it still can’t be seen as a “forum” with “uploading image features”?
Because most of the stuff that gets posted are images instead of text? Who knows, who cares. Semantics.
Ah.
As for me, I asked because I’m pondering over an age-old internet question.
Also:
They say a picture is worth a thousand words, but nobody agrees on the same thousand.
ImageBoards are silly gimmicks for kids to entertain themselves, forums are for serious conversation and organized information.
Most respectable websites lean to forums because they want to be taken seriously. Imagine JGO as an image board, I mean seriously? Who would want that? 
Why are picture magazines not as common as books?