Why are some types from std not in the core of the language?

[mesni]

D has AA and arrays built into the language and runtime. But why 
are sumtype, arrays/smart pointers using custom allocators, 
nullable, tuple at the expense of std? Everything listed in this 
article https://dlang.org/articles/builtin.html applies to them. 
I think this can be done as something like an optional extension 
in the compiler.