C# Uri? vs. Nullable<Uri>

Question

I've always known in C# that you can represent nullable types in multiple ways, with the difference often being described as "syntactic sugar". For example, the following...

// Case 1
int? nullableIntegerVar01;
// Case #2 - 
System.Nullable<int> nullableIntegerVar02;
// Case #3 - 
Uri? nullableUriVar01;
// Case #4 
System.Nullable<Uri> nullableUriVar02;

Oddly, case #4 above does not compile and throw the following error.

"Error CS0453 The type 'Uri' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable'"

I understand the error, I do not understand the reasoning - I expected these two methods of defining nullable types to be interchangeable.

Further, the following does not work.

var testType = typeof(Uri);
var testNullableType = typeof(Nullable<>).MakeGenericType(testType);

Can anyone provide insight?