ints are used in the definition of some of the core interfaces. This has implications on how many entries a collection (that implements these interfaces) can contain.

For example, sort.Interface:

  package sort

  // A type, typically a collection, that satisfies sort.Interface can be
  // sorted by the routines in this package.  The methods require that the
  // elements of the collection be enumerated by an integer index.
  type Interface interface {
  	// Len is the number of elements in the collection.
  	Len() int
  	// Less returns whether the element with index i should sort
  	// before the element with index j.
  	Less(i, j int) bool
  	// Swap swaps the elements with indexes i and j.
  	Swap(i, j int)
  }

It would have implications on array indices as well, since they are also int.