Why can you combine constexpr with const? [duplicate]

This question already has an answer here:

• Difference between `constexpr` and `const` 3 answers

The draft C++11 standard states that constexpr will declare an object as const:

7.1.5p9

A constexpr specifier used in an object declaration declares the object as const. Such an object shall have literal type and shall be initialized.

So it seems that constexpr const int i = 5; is redundant. However, combining const is explicitly disallowed:

7.1.6p2

const can be combined with any type specifier except itself.

Is there any reason why this rule wasn't extended to constexpr?