That is probably because Picard can create and upload AcoustIDs, meaning anyone can do it. Whereas the ISRC’s rely on a geeky script only understood by a few. And it is likely you need to get your physical CDs back out of storage to do this.
A Question for you Digital Submitters
When adding a Release in the CD world, if two CDs hold identical music, have identical artwork, but have a different barcode on the rear, then these are added as two different releases.
Why is it that this is not happening in the Digital world? Some tracks are released direct from the artist website. Without barcodes.
Then later Bandcamp Deezer starts selling those tracks, and adds a barcode. Why is this not a different copy? Why is it that I see editors changing the band released version and adding a barcode to it that was not there at release?