Short answer: because the Standard says so.
Longer answer: prior to Standardization, C++ templates required the class keyword for all template parameters. However, to stress the fact that templates could also be of non-class (i.e. builtin) type, an alternative keyword typename was introduced. However, in C++98, template-template parameters could only be of class-type, and this was the reason that the typename keyword was not added in that context.
Enter C++11 and its new feature template aliases, that now also introduced non-class templates, and hence non-class template-template parameters:
template<typename T> struct A {};
template<typename T> using B = int;
template<template<typename> class X> struct C;
C<A> ca; // ok
C<B> cb; // ok, not a class template
template<template<typename> typename X> struct D; // error, cannot use typename here
The above example was taken from the current C++1z proposal N4051 titled Allow typename in a template template parameter, and proposes to allow precisely that.
Clang 3.5 SVN now supports this with the -std=c++1z flag.