When people receive lots of emails, email starts to break down. Email only works when people aren't flooded with lots of emails all the time.
Because of the high amount of emails people would receive, people miss out on time-sensitive emails, important emails, accidentally delete important emails, forget to read emails, and receive the same questions over and over again, among other problems.
Using a forum solves this problem, plus also allowing features that faciliate discussion. You can categorise topics, link to them, show them publicly, mark them as solved, and have the community also help out.
Could you imagine if Stack Overflow wasn't a forum, and operated entirely by email? How disastrous and frustrating would that be?