Because neither the Democrats or the Republicans could muster the 60 votes needed to get a court-packing scheme into law.
IFAIK the senate can change its rules to eliminate the 60 votes required. After that, there would be no limit on the number of justices in scotus in new bills passed by a simple majority.
If Biden won in 2020 and the senate flipped (dems + rinos), I doubt he would veto such bills. With a dem VP, all he would need would be a 50-50 tie vote with his VP providing the tie breaker. Not likely, but strange things happen.