They do have good things to say about it, but the people who have already made their minds up -- on both sides -- have little reason to listen to it. Instead, they focus on the negativity of either campaign and seize on that as a rallying cry.
More generally, the "Stay in" campaign is about pragmatism, and it's only really the "Leave" campaign that can get really passionate -- because, apparently, this is our golden chance to rid ourselves of a tyrannical despotic regime, and emerge triumphant in the free new world, etc etc.