Why is the ctor of optional<T&> explicit?

I mean the ctor `template explicit optional(const optional& rhs)`. It doesn't seem to have much sense - optional is logically a reference and it seems reasonable to allow it to be converted implicitly if the underlying pointer types allow this.
With the current implementation the usage of optional references can be cumbersome, because even the non-const-ref to const-ref conversion has to be explicit.