The committee got burned, extremely badly, by C++03's export templates, where it got standardized without an implementation, and everyone realized it was basically unimplementable, and the only group that did, wrote a paper telling everyone not to [1]

This was well in mind when C++11 came around (especially since C++11 slipped so badly: it was originally "C++0x", and then they ran out of digits for x). By the time we hit C++20, I think the lessons were lost, especially when it came to there being two competing modules implementations and the committee deciding to select neither.

1: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n14...