It was the same when the galaxy buds were the freebie on other phone promotions @AndrewAR4
i.e you could only choose the white one's and not the black versions.
It's a business decision by Samsung I assume.
If the Mystic Bronze isn't the colour you want then you could as an option have them sent and then sell them to help pay for the Mystic Black versions !
Just a thought.
🌈 Stay Safe & Stay Alert. 🌈
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.