For the most part, only that nobody has proposed or tried to implement them. One issue would be that static variables in generic contexts would likely need to be instantiated for each set of generic arguments, which would require additional runtime support similar to what we'd need to support static vars in generic types.
It also stretches the meaning of static, which currently means "associated with a type instead of an instance". (Admittedly the only reason it means this is because of C++ going the other direction, from the original C static-lifetime-as-opposed-to-auto to static-as-opposed-to-instance, but…)
It does, but C++ is a language where many keywords or symbols have multiple meanings based on context. (C++ has many positive features but I don't consider this one of them!)