If ${/* ... */} were allowed, it would be reasonable to expect that ${} was also allowed, since a comment is supposed to just remove itself. If we don't do that, we'd be adding some inconsistencies to the comment syntax.
Now, what would it mean to do ${}? It sounds like you would want that to effectively mean the same thing as ${''}, i.e. if you omit the expression between ${...}, then it defaults to an empty string. This basically makes ${/* ... */} a convenient shorthand for ${''/* ... */} which is certainly a possibility.
As for adding comments to regular expressions, there's a whole entire thread over here devoted to improving regular expressions via comment support. The discussion lead to this proposal repo - so far there aren't any champions who are wanting to push it, but that seems to be the current agreed upon solution from that thread if we wanted to solve this problem.