I think the reason that ISRCs are stored and not many other non-MB IDs, is that there’s a standard field for ISRCs. For more freeform tagging formats like Vorbis Comments, this would not be a hindrance, but for other formats that have a limited/defined set of valid tags, it may not be possible to properly just add the ISRC.
Also, just for the record, note that ISRCs are not unique identifiers. One ISRC can have been used for multiple recordings, and a single recording can have been assigned multiple ISRCs. This is, and was, not the intention of the ISRC system, but never the less it’s how it’s turned out.