When creating a new programming language, there needs to be a standard library for the programming language to be useful.

The standard library (and/or core library, like in Rust or Ruby) contains all the useful things such as filesystem access, collections such as arrays and dictionaries, networking, and more.

This is probably a problem inherent to creating a programming language, as each language has its idioms, and a standard library would best use these idioms.

See also: Writing cross-language implementation is difficult.