Why can't a string be nil in Go?

The simple answer is that nil is not defined to be a valid value for type string in the language specification.

...but maybe you want a longer answer?

nil is the zero value for pointers, interfaces, channels, slices, maps and function types, and it represents an uninitialized state.

Consider the following variable declarations:

var a *SomeType
var b interface{}
var c func()

It seems natural that all these variables would have a value that represents uninitialized state. a has been declared as a pointer, but what would it point to, when we haven't yet pointed it at anything? nil is an obvious zero value for these types.

As for channels, slices and maps, their zero value is nil for the simple reason that their implementation is such that they must be explicitly initialized before they can be used. This is mostly for performance reasons, these types are all represented internally as more or less complex data structures, and initializing them is not free.

However, a string doesn't require initialization, and it seems natural that the default, zero value for a new string variable would be an empty string, "". Therefore there's simply no reason for nil to be a valid string value, and adding it to the specification would only make the language more complicated and much more cumbersome to work with.

Furthermore, what would nil of type string represent? An empty string? We already have that. An uninitialized string? There's no such thing.

In go string is a data type, it's no a pointer to an array like C/C++. As such you can't assign it to nil.