Why aren't Ranges Interfaces?

[Jack Stouffer via Digitalmars-d-learn]

I have been learning D over the past three weeks and I came to 
the chapter in "Programming in D" on Ranges. And I am a little 
confused on the choice to make Ranges based on the methods you 
have in the struct, but not use a interface. With all of the 
isInputRange!R you have to write everywhere, it just seems like 
it would have made a lot more sense and made everyone's jobs 
easier if the different types of Ranges where just interfaces 
that you could inherit from.

The only reason I can think of to not do it this way is the weird 
distinction between structs and classes in D.