I only try to imagine Larian perspective on it. I would like to join zhentarim
Their whole design is that there are no "real" side quests. If you look at it, every side activity connects with the main plot somehow. For example, you want to help barcus wroot. Then you learn, that his friend can be kept somewhere in the Underdark. Then you learn that grymforge is full of gnome slaves, but you need to go there nonetheless. Then you learn that barcus's friend was sent to moonrise towers, and you need to go there for the main plot.
And every side quest arc does the same thing. So only if Larian will somehow justify you joining a faction for the main plot, they won't let you do it otherwise
