There is a previous paper, n3499, which tell us that although Bjarne himself suggested spaces as separators:
While this approach is consistent with one common typeographic style, it suffers from some compatibility problems.
- It does not match the syntax for a pp-number, and would minimally require extending that syntax.
- More importantly, there would be some syntactic ambiguity when a hexadecimal digit in the range [a-f] follows a space. The preprocessor would not know whether to perform symbol substitution starting after the space.
- It would likely make editing tools that grab “words” less reliable.
I guess the following example is the main problem noted:
const int x = 0x123 a;
though in my opinion this rationale is fairly weak. I still can’t think of a real-world example to break it.
The “editing tools” rationale is even worse, since 1'234 breaks basically every syntax highlighter known to mankind (e.g. that used by Markdown in the above question itself!) and makes updated versions of said highlighters much harder to implement.
Still, for better or worse, this is the rationale that led to the adoption of apostrophes instead.