[Jtsummers]

C++ implementations were commercial products. Same with C before it, and many (possibly most) languages used in industry. Because platforms were less uniform, and even where they were porting still took much more effort than it does today.

It's really hard to justify making a new go, python, lua, etc. implementation when the primary ones are stable and support a large number of platforms.

And adding language features by altering syntax/semantics, you might as well produce an entire new language rather than deal with the headaches of being partially compatible and having to track the original over time.