@leocg Several reasons (from my personal experience at least):
From my experience forums often do a rather poor job of preventing duplicates. Forums can be intimidating / too much of a hassle to new users / people that have hardly used a forum A forum has not the necessary features to determine actual user demand and therefore can only somewhat help to prioritise tasks Forums often require someone to actually write something, leaving out the large amount of lurkers on the internet The search is most of the times not as goodWhilst forums might be better for discussing things, when it comes to uncomplicated feedback, Uservoice (or a similar system) are far superior.
A lot of tech-giants use it. Microsoft uses it for almost every single one of their products (though they started implementing their own version of such a system in windows, namely the "feedback hub" or something along those lines): OneDrive, Xbox, Xbox Music, Visual Studio, Cortana.
OneDrive: https://onedrive.uservoice.com/forums/262982-onedrive
Adobe uses it for their new experience design application. And for the services I have used it has been quite great.
Adobe XD: https://adobexd.uservoice.com