“But why should one have to ask his LOR whether a particular kind of tallis katan is acceptable? Why can’t a minimal standard be generally accepted by Jews everywhere?”
Because this is a matter of debate between the poskim (e.g. The Mishnah Berurah and the Chazon Ish). Also, since there exists mass produced taleisim (gadol and koton) whose products do not meet even the basic standards (placement of the holes, sqaureness of the corners, size of the shoulders versus the hole, etc.
