The problem is that the wide majority of them were (and still are) conspiracy nuts
Instead, it leaves the parties on the fringe as the only ones doing the criticizing.
Why should it be controversial to say that the EU is not perfect and that leaving it could work out better for certain countries?
Instead it's like no one can imagine a time where the EU did not exist and it's almost like we have forgotten that the EU is not the end all be all of everything.
In the best case when it doesn't work well it's a convenient target to blame (when it's their own fault).
In other cases they'd like the freedom to be *more* (much more) authoritarian than EU membership allows.
But currently let's be honest, most simply work for Putin or China and the aim is just to wreak havoc.