Initialization of static const member variables is available for integral and enum types. This feature existed in C++ since the beginning of times. It is needed to facilitate usage of static const members in integral constant expressions (i.e. as compile-time constants), which is an important feature of the language. The reason integral and enum types were singled out and treated in this exceptional fashion is that integral constants are often used in compile-time contexts, which require no storage (no lvalue) for the constant.

The ability to supply initializers for non-static members is a new (for C++11) feature. It is a completely different feature, albeit it looks similar at syntax level. Such initializers provide default values for compiler-provided constructors.

In other words, it is not correct to lump these two features (initializers for static and non-static members) together. These two features are completely different. They are based on completely unrelated internal mechanics. Your question essentially applies the first feature: how come non-const static members cannot be initialized in-class? It is basically a C++98 question and the most likely answer to it is that there was never any reason to treat non-const static members in such exceptional way. Non-const static members are treated in accordance with the general rules: they require a separate definition and the initializer should be provided at the point of definition.

Initialization of static const member variables is available for integral and enum types. This feature existed in C++ since the first language standard (C++98). It is needed to facilitate usage of static const members in integral constant expressions (i.e. as compile-time constants), which is an important feature of the language. The reason integral and enum types were singled out and treated in this exceptional fashion is that integral constants are often used in compile-time contexts, which require no storage (no definition) for the constant.

The ability to supply initializers for non-static members is a new (for C++11) feature. It is a completely different feature, even though it looks similar at syntax level. Such initializers are used as construction-time initializers for those class members that were not explicitly initialized by the user.

In other words, it is not correct to lump these two features (initializers for static and non-static members) together. These two features are completely different. They are based on completely unrelated internal mechanics. Your question essentially applies the first feature: how come non-const static members cannot be initialized in-class? It is basically a C++98 question and the most likely answer to it is that there was never any reason to treat non-const static members in such an exceptional way. Non-const static members are treated in accordance with the general rules: they require a separate definition and the initializer should be provided at the point of definition.