Wouldn't it be more likely that maintaining backwards compatibility (an important Windows trait, after all) while also implementing all of Powershell would be a considerably more difficult task than just making a new scripting environment?
yes, too much to ask for a multi-trillion company, especially that this company was until recently well known for maintaining crazy level of backward compatibility
But… this is backwards compatibility. The command prompt is backwards compatible. Sometimes the best way to ensure that is to preserve in amber and replace.