Basically, quantified types can't be given as arguments to type
constructors (other than ->, which is its own thing). I'm not entirely sure
why, but it apparently makes the type system very complicated from a
theoretical standpoint. By wrapping the quantified type in a newtype, the
argument to IO becomes simple enough not to cause problems.

GHC has an extension -XImpredicativeTypes that lifts this restriction, but in my experience, it doesn't work very well.