Difference between initialization of static variables in C and C++

It compiles in C++ because C++ needs to support dynamic initialization anyway, or you couldn't have local static or non-local objects with non-trivial constructors.

So since C++ has this complexity anyway, supporting that initialization like you show isn't complicated to add anymore.

In C that would be a big matter because C doesn't have any other reason to support initialization done at program startup (apart from trivial zero initialization). In C, initial values of file-scope or local static objects can always statically be put into the executable image.

6.7.8/4 [C99]

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.

In static int i = initializer(); the RHS is not a constant expression and so the code doesn't compile in C.

In C++ there is no such restriction and the code is well-formed in C++.

Static variables in C need to be initialised with a value known at compile time. This requirement has been removed in C++, and you can initialise them with expressions evaluated at run-time.

The two languages differ in this, and many, many other respects. You can quite easily write C code which will be acceptable to a C++ compiler, but the reverse is not true.