Why is that?
There is a list of prohibited buyers, not a list of allowed buyers.
Anyone who cannot be trusted to own and carry arms should not be running around loose!
“Why is that?” (a universal background check would have implement universal registration for it to be possible.)
Think about it. How do you enforce the requirement that private sellers have to participate in a background check?
The only way it will work is for the government to maintain a record of what guns you own and a record of what you bought and sold. Every state that has a universal background checks has registration.
BTW, this universal background check is the old gun show loophole. Their last attempt to close it was designed to impose such legal liabilities upon gun show promoters as to put them out of business.