Why constexpr data members are not implicitly static?

if you do this:

constexpr int LEN = 100;

LEN variable defined as const without need of typing const keyword.
It also have static storage, without need to type static keyword.

From the other hand, if we do same in class:

struct A{
   constexpr static int SIZE = 100;
};

SIZE is still defined as const without need of typing const keyword,

However SIZE is not static data member.
You need to type static explicitly. If you don't there will be compilation error.

Question is:
What is the reason of need to explicitly type static?